Well if you worked in retail, or on a production line, or picking fruit, or on a building site, you wouldn't have a nice glass of water on your desk to stop you getting a headache. You'd just drink a good amount each time you had the opportunity and probably be absolutely fine. I refuse to believe that the whole of the UK before the 1990s was perpetually dehydrated!
Children in our primary school have the following opportunities to drink, and there are 5 water fountains between 250:
On arrival at school 8.15
On way to class after assembly 8.55
Break 10.20-10.45 (jugs of water and cups provided as well as water fountains)
Lunch 12 - 1.15 (in the playground, and at lunch, of course)
Mid afternoon at around 2.30
Going home time 3.30
Teachers mostly allow kids to get water in between if they're really thirsty.
I really think that's absolutely fine. They are never going more than an hour without a chance to drink.
Water bottles are sent in by some parents. They are rarely used. They fester in bags, lockers etc. In this weather the water is warm and horrible by mid morning. In my DD's previous school this fanaticism about water meant that they had it on desks at all times and frequently it made a right old mess and was a constant distraction.
If your school is not providing enough drinking facilities for every kid to get a good drink at break and lunch, lobby them to do better. It's absolutely true that water is a basic right for kids in school but I just don't agree that it all requires this degree of fuss.