diff --git a/backend/app.py b/backend/app.py
index 4828a8b..d352de5 100644
--- a/backend/app.py
+++ b/backend/app.py
@@ -808,7 +808,13 @@ async def get_school_details(request: Request, urn: int):
         "census": supplementary.get("census"),
         "admissions": supplementary.get("admissions"),
         "admissions_history": supplementary.get("admissions_history") or [],
-        "admission_distance": supplementary.get("admission_distance"),
+        # Behind a flag, and withheld at the source rather than rendered-but-
+        # hidden: this endpoint is public and unauthenticated, so a field left
+        # in the payload is a published field. The key is absent, not null —
+        # null would state that this school has no cut-off, which is a
+        # different claim from "we are not publishing cut-offs".
+        **({"admission_distance": supplementary.get("admission_distance")}
+           if flags.is_enabled("admission_distance") else {}),
         "sen_detail": supplementary.get("sen_detail"),
         "phonics": supplementary.get("phonics"),
         "deprivation": supplementary.get("deprivation"),
diff --git a/backend/tests/test_flags.py b/backend/tests/test_flags.py
index 114a219..24fdb53 100644
--- a/backend/tests/test_flags.py
+++ b/backend/tests/test_flags.py
@@ -93,3 +93,71 @@ def test_the_flags_endpoint_answers_false_when_unleash_is_unreachable(monkeypatc
     res = _client().get("/api/flags")
     assert res.status_code == 200
     assert all(v is False for v in res.json().values())
+
+
+def _school_payload(monkeypatch, *, flag_on: bool):
+    """Fetch one school's payload with the distance flag forced on or off.
+
+    The DataFrame shape is copied from test_school_details.py rather than
+    minimised: the endpoint reads a wide set of GIAS columns, and a trimmed
+    frame fails for reasons that have nothing to do with flags.
+    """
+    import numpy as np
+    import pandas as pd
+    from fastapi.testclient import TestClient
+    from backend import app as app_module
+
+    df = pd.DataFrame([{
+        "urn": 150275,
+        "school_name": "West London Performing Arts Academy",
+        "phase": "Secondary",
+        "school_type": "Special post 16 institution",
+        "trust_name": None,
+        "religious_denomination": "Does not apply",
+        "gender": None,
+        "age_range": "16-25",
+        "admissions_policy": None,
+        "capacity": np.nan,
+        "gias_total_pupils": np.nan,
+        "headteacher_name": None,
+        "website": None,
+        "ofsted_grade": np.nan,
+        "local_authority": "Ealing",
+        "address": "268 Northfield Avenue, London, W5 4UB",
+        "postcode": "W5 4UB",
+        "latitude": 51.4986,
+        "longitude": -0.3148,
+        "year": np.nan,
+        "total_pupils": np.nan,
+        "eligible_pupils": np.nan,
+        "rwm_expected_pct": np.nan,
+    }])
+
+    monkeypatch.setattr(app_module, "load_school_data", lambda: df)
+    # Two arguments: get_supplementary_data(db, urn). See backend/app.py.
+    monkeypatch.setattr(
+        app_module, "get_supplementary_data",
+        lambda db, urn: {"admission_distance": {"distance_m": 772.49,
+                                                "year": 2024}})
+    monkeypatch.setattr(flags, "is_enabled", lambda name: flag_on)
+
+    client = TestClient(app_module.app, raise_server_exceptions=False)
+    res = client.get("/api/schools/150275")
+    assert res.status_code == 200, res.text
+    return res.json()
+
+
+def test_the_distance_field_is_absent_when_the_flag_is_off(monkeypatch):
+    """Absent, not null, and withheld at the source.
+
+    /api/schools/ is public and unauthenticated. Leaving a withheld field in
+    the payload while declining to render it hands the record to anyone who
+    opens the network tab — the reasoning already recorded in c9a1892.
+    """
+    body = _school_payload(monkeypatch, flag_on=False)
+    assert "admission_distance" not in body
+
+
+def test_the_distance_field_is_present_when_the_flag_is_on(monkeypatch):
+    body = _school_payload(monkeypatch, flag_on=True)
+    assert body["admission_distance"]["distance_m"] == 772.49
diff --git a/nextjs-app/lib/types.ts b/nextjs-app/lib/types.ts
index 7c1f751..601fd1f 100644
--- a/nextjs-app/lib/types.ts
+++ b/nextjs-app/lib/types.ts
@@ -361,7 +361,12 @@ export interface SchoolDetailsResponse {
    * held back as a paid feature and are not part of this public payload — see
    * data_loader._admission_distance.
    */
-  admission_distance: SchoolAdmissionDistance | null;
+  /**
+   * Absent — not null — when the admission_distance flag is off. Null means
+   * "this school has no published cut-off"; absent means "cut-offs are not
+   * being published at all". They are different claims and the type says so.
+   */
+  admission_distance?: SchoolAdmissionDistance | null;
   deprivation: SchoolDeprivation | null;
   finance: SchoolFinance | null;
 }