fix(compare): per-school P8 explanation; disadvantaged cohort from the same yearly row

Review findings: (1) the 'no KS2 baseline (COVID)' explanation was derived
from the basket-wide max year, so a school with an unrelated data gap
borrowed it from a neighbour with 2024/25 data — now judged per school on
its own latest year. (2) The '~N disadvantaged pupils' cohort multiplied
eligible_pupils and disadvantaged_pct resolved independently across years
— now both come from the same yearly row that supplies the displayed
percentage.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/components/compare/CompareAcademics.tsx

@@ -150,13 +150,15 @@ export function CompareAcademics({
     });
     // DfE stopped publishing Progress 8 from 2024/25: those GCSE year groups
     // sat no KS2 tests (COVID), so there is no baseline to measure progress
-    // from. A bare "No data" reads as a gap on our side — say why.
-    const latestYear = urns.reduce((max, urn) => {
+    // from. A bare "No data" reads as a gap on our side — say why. Judged
+    // PER SCHOOL on its own latest data year: a school whose data simply
+    // stops earlier (an unrelated gap) must not borrow the COVID explanation
+    // from a neighbour that does have 2024/25 data.
+    const p8NotPublished = urns.map((urn) => {
       const rows = data[String(urn)]?.yearly_data ?? [];
       const y = rows.length ? Math.trunc(rows[rows.length - 1].year) : 0;
-      return Math.max(max, y);
-    }, 0);
-    const p8NotPublished = latestYear >= 202425;
+      return y >= 202425;
+    });
     const grade5 = latestValues(data, urns, 'english_maths_strong_pass_pct');
     const ebacc = latestValues(data, urns, 'ebacc_entry_pct');
     const att8Anchor = nationalAverages?.secondary?.attainment_8_score;
@@ -198,7 +200,7 @@ export function CompareAcademics({
                 >
                   {banding[i]}
                 </Chip>
-              ) : p8NotPublished ? (
+              ) : p8NotPublished[i] ? (
                 <span className={s.small}>
                   Not published — this GCSE year group sat no KS2 tests (COVID), so DfE has no
                   baseline to measure progress from
@@ -234,15 +236,21 @@ export function CompareAcademics({
   const disadvantagedAnchor = benchmarks?.primary?.disadvantaged_rwm_expected_pct ?? null;
   // Cohort size behind the disadvantaged figure (spec §8.5): these are small
   // groups where single pupils move the percentage — show roughly how many
-  // pupils the figure rests on.
-  const eligible = latestValues(data, urns, 'eligible_pupils');
-  const disadvantagedShare = latestValues(data, urns, 'disadvantaged_pct');
-  const cohorts = urns.map((_, i) => {
-    const n = eligible[i];
-    const share = disadvantagedShare[i];
-    if (n == null || share == null) return null;
-    const cohort = Math.round((n * share) / 100);
+  // pupils the figure rests on. Taken from the SAME yearly row that supplies
+  // the displayed percentage: resolving eligible_pupils and the
+  // disadvantaged share independently could mix years and misstate the
+  // cohort behind the figure.
+  const cohorts = urns.map((urn) => {
+    const rows = data[String(urn)]?.yearly_data ?? [];
+    for (let i = rows.length - 1; i >= 0; i--) {
+      const row = rows[i];
+      if (row.rwm_expected_disadvantaged_pct != null) {
+        if (row.eligible_pupils == null || row.disadvantaged_pct == null) return null;
+        const cohort = Math.round((row.eligible_pupils * row.disadvantaged_pct) / 100);
         return cohort > 0 ? cohort : null;
+      }
+    }
+    return null;
   });
 
   return (