fix(api): treat "16 plus" as secondary, the way PHASE_GROUPS already does

GIAS phase 6 is "16 plus", and PHASE_GROUPS deliberately files it in the
secondary group. The payload helper decided the same question with
`"secondary" in phase_text`, which that value does not satisfy — so a
sixth-form college was handed the primary bucket and offered infant schools
as its peers, with the KS2 metric key to label them. No crash; just a page
confidently showing the wrong schools.

The decision now lives in similar_schools.is_secondary_phase, beside the
PHASE_GROUPS bucket it selects from, so the two cannot drift again. A test
pins them together.

The same binary assumption had a second output. computeSchoolFlags tests for
the substring too, so a 16-plus school renders the primary template, and the
composer was labelling the section from the template: "Other primary schools
near <sixth form college>" above a row of secondaries. The section now
derives its noun from the school's own phase, which also removes the
duplicated wording from both composers. A 16-plus school's candidates span
the whole secondary group, so no single noun fits and it gets the honest
general one.

Reported in review on #150.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app.py

@@ -41,7 +41,7 @@ from .data_loader import get_data_info as get_db_info
 from . import flags
 from .places import build_place_index, build_place_registry, places_for_urn
 from .schemas import METRIC_DEFINITIONS, PHASE_GROUPS, RANKING_COLUMNS, SCHOOL_COLUMNS
-from .similar_schools import select_similar
+from .similar_schools import is_secondary_phase, select_similar
 from .utils import clean_for_json, convert_to_native
 
 # Values to exclude from filter dropdowns (empty strings, non-applicable labels)
@@ -274,11 +274,12 @@ def _similar_schools_payload(urn: int, phase: str | None) -> list[dict]:
     section simply does not render.
     """
     try:
-        phase_text = (phase or "").lower()
+        # Decided in similar_schools, beside the PHASE_GROUPS bucket it selects
-        # All-through schools render with the primary template, which is what
+        # from, so the two cannot drift. A substring test for "secondary" here
-        # decides the metric, so they are not secondary here.
+        # would miss "16 plus" and hand a sixth-form college the primary bucket.
-        is_secondary = phase_text != "all-through" and "secondary" in phase_text
+        return select_similar(
-        return select_similar(load_latest_school_data(), int(urn), is_secondary)
+            load_latest_school_data(), int(urn), is_secondary_phase(phase)
+        )
     except Exception:
         import logging
 

backend/similar_schools.py

@@ -89,6 +89,24 @@ def phase_label(phase: str | None) -> str:
     return f"{text.capitalize()} school"
 
 
+def is_secondary_phase(phase: str | None) -> bool:
+    """Whether this phase takes the secondary side: secondary group membership,
+    minus all-through.
+
+    Membership is read from PHASE_GROUPS rather than tested with `"secondary" in
+    phase`, because that substring misses "16 plus" — GIAS phase 6, which
+    PHASE_GROUPS deliberately files as secondary. The substring version fails
+    silently rather than loudly: a sixth-form college is simply handed the
+    primary bucket and offered infant schools as peers.
+
+    All-through is the exception. PHASE_GROUPS lists it on both sides because it
+    belongs on both phases' place pages, but the detail page renders it with the
+    primary template, and the metric follows the template.
+    """
+    text = (phase or "").strip().lower()
+    return text != "all-through" and text in PHASE_GROUPS["secondary"]
+
+
 def _phase_group(is_secondary: bool) -> set[str]:
     return PHASE_GROUPS["secondary" if is_secondary else "primary"]
 

backend/tests/test_similar_schools.py

@@ -11,7 +11,7 @@ set, never far enough to fill the last of the six slots.
 import numpy as np
 import pandas as pd
 
-from backend.similar_schools import select_similar
+from backend.similar_schools import is_secondary_phase, select_similar
 
 # Roughly 0.7 miles apart in latitude at this longitude.
 BASE_LAT, BASE_LON = 51.5000, -0.1000
@@ -203,6 +203,38 @@ def test_all_through_is_offered_on_both_phase_sides():
     assert 100002 in {s["urn"] for s in select_similar(secondary, 100010, is_secondary=True)}
 
 
+def test_sixteen_plus_is_matched_against_secondary_not_primary():
+    """GIAS phase 6 is "16 plus", and PHASE_GROUPS puts it in the secondary
+    group — a sixth-form college's peers are secondaries and other colleges,
+    never primary schools. A substring test for "secondary" misses it silently:
+    no crash, just a page offering infant schools to a sixth form."""
+    frame = _frame(
+        _row(100001, "Sixth Form College", phase="16 plus", age_range="16-19"),
+        _row(100002, "Nearby Secondary", phase="Secondary", latitude=_at(0.5),
+             attainment_8_score=52.0),
+        _row(100003, "Nearby College", phase="16 plus", latitude=_at(0.6),
+             attainment_8_score=np.nan),
+        _row(100004, "Nearby Primary", phase="Primary", latitude=_at(0.1)),
+    )
+    result = select_similar(frame, 100001, is_secondary=is_secondary_phase("16 plus"))
+    urns = {s["urn"] for s in result}
+    assert 100004 not in urns, "a primary school is not a peer for a sixth form"
+    assert urns == {100002, 100003}
+    assert all(s["metric_key"] == "attainment_8_score" for s in result)
+
+
+def test_is_secondary_phase_agrees_with_the_phase_groups_it_selects_from():
+    """The two must not drift: whatever this calls secondary decides which
+    PHASE_GROUPS bucket the candidates come from."""
+    for phase in ("Secondary", "Middle deemed secondary", "16 plus"):
+        assert is_secondary_phase(phase) is True, phase
+    for phase in ("Primary", "Middle deemed primary", "Nursery", "", None):
+        assert is_secondary_phase(phase) is False, phase
+    # In PHASE_GROUPS an all-through school is on both sides, but it renders
+    # with the primary template, and the metric follows the template.
+    assert is_secondary_phase("All-through") is False
+
+
 def test_chips_state_only_what_the_tier_earned():
     frame = _frame(
         _row(100001, "Subject", phase="Secondary", gender="Mixed",

nextjs-app/__tests__/components/SimilarSchoolsSection.test.tsx

@@ -8,6 +8,7 @@
 import { render, screen } from '@testing-library/react';
 
 import {
+  nearbyNoun,
   SimilarSchoolsSection,
   shouldRenderSimilar,
 } from '@/components/school/SimilarSchoolsSection';
@@ -46,7 +47,7 @@ function renderSection(similar: SimilarSchool[]) {
     <SimilarSchoolsSection
       urn={100001}
       schoolName="Meadowbrook Primary School"
-      phaseNoun="primary"
+      phase="Primary"
       thisMetricValue={72}
       similar={similar}
     />,
@@ -85,6 +86,37 @@ describe('the claim the lede makes', () => {
   });
 });
 
+describe('what the lede calls the set', () => {
+  it.each([
+    ['Primary', 'primary schools'],
+    ['Middle deemed primary', 'primary schools'],
+    ['Secondary', 'secondary schools'],
+    ['Middle deemed secondary', 'secondary schools'],
+    ['All-through', 'all-through schools'],
+    // GIAS phase 6. Its candidates span the whole secondary group, so no
+    // single noun fits and it takes the honest general one.
+    ['16 plus', 'schools and colleges'],
+    ['', 'schools'],
+    [null, 'schools'],
+  ])('calls a %s school\'s neighbours "%s"', (phase, expected) => {
+    expect(nearbyNoun(phase)).toBe(expected);
+  });
+
+  it('never calls a sixth form college\'s neighbours primary schools', () => {
+    render(
+      <SimilarSchoolsSection
+        urn={100001}
+        schoolName="Barnet Sixth Form College"
+        phase="16 plus"
+        thisMetricValue={null}
+        similar={[school(), school({ urn: 100003 })]}
+      />,
+    );
+    expect(screen.getByText(/Other schools and colleges near Barnet Sixth Form College/)).toBeInTheDocument();
+    expect(screen.queryByText(/primary schools/)).not.toBeInTheDocument();
+  });
+});
+
 describe('cards', () => {
   it('links each school to its canonical slug', () => {
     renderSection([school(), school({ urn: 100003, school_name: 'Oakfield Primary School' })]);

nextjs-app/components/school/PrimarySchoolSections.tsx

@@ -155,7 +155,7 @@ export function PrimarySchoolSections({
       <SimilarSchoolsSection
         urn={schoolInfo.urn}
         schoolName={schoolInfo.school_name}
-        phaseNoun={flags.isAllThrough ? 'all-through' : 'primary'}
+        phase={schoolInfo.phase}
         thisMetricValue={flags.latestResults?.rwm_expected_pct ?? null}
         similar={similarSchools}
       />

nextjs-app/components/school/SecondarySchoolSections.tsx

@@ -149,7 +149,7 @@ export function SecondarySchoolSections({
       <SimilarSchoolsSection
         urn={schoolInfo.urn}
         schoolName={schoolInfo.school_name}
-        phaseNoun="secondary"
+        phase={schoolInfo.phase}
         thisMetricValue={flags.latestResults?.attainment_8_score ?? null}
         similar={similarSchools}
       />

nextjs-app/components/school/SimilarSchoolsSection.tsx

@@ -28,6 +28,28 @@ export function shouldRenderSimilar(similar?: SimilarSchool[] | null): boolean {
   return (similar?.length ?? 0) >= MINIMUM;
 }
 
+/**
+ * What the lede calls the set of schools it is showing.
+ *
+ * Derived from the school's own GIAS phase rather than the template it renders
+ * with, because those disagree for "16 plus" (GIAS phase 6): a sixth-form
+ * college renders the primary template — computeSchoolFlags tests for the
+ * substring "secondary" — while the backend correctly matches it against the
+ * secondary group. Taking the noun from the template would print "Other primary
+ * schools near <sixth form college>" above a row of secondaries.
+ *
+ * A 16-plus school's candidates span the whole secondary group, so no single
+ * noun fits and it gets the honest general one.
+ */
+export function nearbyNoun(phase: string | null | undefined): string {
+  const text = (phase ?? '').trim().toLowerCase();
+  if (text === 'all-through') return 'all-through schools';
+  if (text === '16 plus') return 'schools and colleges';
+  if (text.includes('secondary')) return 'secondary schools';
+  if (text.includes('primary')) return 'primary schools';
+  return 'schools';
+}
+
 function metricLabel(key: string): string {
   return key === 'attainment_8_score' ? 'Attainment 8' : 'Reading, writing & maths';
 }
@@ -40,13 +62,14 @@ function formatMetric(value: number | null, key: string): string {
 export function SimilarSchoolsSection({
   urn,
   schoolName,
-  phaseNoun,
+  phase,
   thisMetricValue,
   similar,
 }: {
   urn: number;
   schoolName: string;
-  phaseNoun: string;
+  /** The school's own GIAS phase, not the template it renders with. */
+  phase: string | null | undefined;
   thisMetricValue: number | null;
   similar?: SimilarSchool[] | null;
 }) {
@@ -57,6 +80,7 @@ export function SimilarSchoolsSection({
   // shares an intake with this school.
   const loosest = Math.max(...schools.map((s) => s.tier));
   const metricKey = schools[0].metric_key;
+  const noun = nearbyNoun(phase);
 
   return (
     <Section id="similar">
@@ -70,8 +94,8 @@ export function SimilarSchoolsSection({
             </h2>
             <p className={styles.lede}>
               {loosest >= 3
-                ? `Other ${phaseNoun} schools near ${schoolName}.`
+                ? `Other ${noun} near ${schoolName}.`
-                : `Other ${phaseNoun} schools near ${schoolName}, with a similar intake.`}
+                : `Other ${noun} near ${schoolName}, with a similar intake.`}
             </p>
           </div>
         }