Borehole water, tanker water, and even county taps can look clean and still fail on bacteria, nitrates, or chlorine.

A head teacher does not need a lab. One kit, one tap, one afternoon:

• Test the tap students actually drink from, not the kitchen sink.

• Record the date and the result. Inspectors ask for this.

• Repeat every week of term. One test in January is not a water programme.

If chlorine is low and the water sits in a tank, bacteria can grow overnight. If nitrates are high, younger children are the ones at risk.

The cheapest time to find a problem is before parents start calling.