Two children drank from the same school tap

Why

The teacher has spotted something genuinely useful: if the water were badly contaminated you would expect both children to fall ill, so one case out of two is a real reason not to panic. It just is not enough to clear the tap. Two people who drink the same water can still end up differently — one may have swallowed more, one may have eaten something else entirely, and bodies differ in how much they can shrug off. Equally, the sick child's cause may have nothing to do with water at all. A single pair of children cannot separate those possibilities. Test: what would settle it is many children over several days, or having the water tested. The conclusion does not hold — the evidence is consistent with safe water AND with mildly unsafe water, and when a fact fits both answers it has not chosen between them.