feat(pipeline): derive dim_school.has_sixth_form from GIAS flag

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

pipeline/transform/models/marts/_marts_schema.yml

@@ -16,6 +16,17 @@ models:
         tests:
           - not_null:
               severity: warn
+      - name: has_sixth_form
+        description: >
+          Authoritative sixth-form flag from GIAS OfficialSixthForm.
+          "Has a sixth form" => true; "Does not have a sixth form" and
+          "Not applicable" => false; blank GIAS value falls back to
+          statutory_high_age >= 18. Replaces the age_range-contains-"18"
+          heuristic (spec 2026-07-07 §3).
+        tests:
+          - not_null
+          - accepted_values:
+              values: [true, false]
       - name: status
         tests:
           - accepted_values:

pipeline/transform/models/marts/dim_school.sql

@@ -52,6 +52,14 @@ select
     s.religious_character,
     s.gender,
     s.statutory_low_age || '-' || s.statutory_high_age as age_range,
+    -- Authoritative sixth-form flag (spec §3): GIAS OfficialSixthForm.
+    -- "Not applicable" (nurseries, primaries, PRUs) => false. Blank GIAS
+    -- value (rare, new establishments) falls back to the statutory age range.
+    case
+        when s.official_sixth_form = 'Has a sixth form' then true
+        when s.official_sixth_form in ('Does not have a sixth form', 'Not applicable') then false
+        else coalesce(s.statutory_high_age >= 18, false)
+    end as has_sixth_form,
     s.capacity,
     s.total_pupils,
     concat_ws(' ', s.head_title, s.head_first_name, s.head_last_name) as headteacher_name,