diff --git a/pipeline/transform/models/marts/_marts_schema.yml b/pipeline/transform/models/marts/_marts_schema.yml
index 0569afc..4291cfb 100644
--- a/pipeline/transform/models/marts/_marts_schema.yml
+++ b/pipeline/transform/models/marts/_marts_schema.yml
@@ -9,7 +9,13 @@ models:
       - name: school_name
         tests: [not_null]
       - name: phase
-        tests: [not_null]
+        description: >
+          Primary / Secondary / All-through etc. May be null for a small number
+          of independent schools where GIAS publishes "Not Applicable", no
+          statutory age range, and the school name gives no hint.
+        tests:
+          - not_null:
+              severity: warn
       - name: status
         tests:
           - accepted_values: