I'm not sure about DD's middle school. She takes a bottle to school, but I'm not sure if she leaves it in her locker during the day. They are on a part-rotary schedule, so it may be her choice not to carry it from class to class during the day. I know the rotary contributes to a fair amount of lost belongings from absent-minded kids, so I wouldn't be surprised to find out that kids were asked to keep bottles in their lockers.
I know she was allowed a bottle at her desk at her primary school, but she spent almost all day in the same class except for gym, music, drama, and library classes.