Our local (primary) school has this as a policy. Forget your water bottle? First warning. Second time - detention. It's part of their 'uniform policy', so apparently forgetting your water bottle is equivalent to wearing the wrong jumper. Which is surely more of a choice than an accidental error.
I get that hydration is important. But this is insane, right? They can't detain my 4 year old for a mistake her parents might have made?