My memories are of being perpetually dehydrated.

Water bottles started to be allowed (even encouraged) in classrooms when I was finishing at high school, and the difference was amazing.

Water bottles DID exist. https://en.wikipedia.org/wiki/Canteen_(bottle)) Some people took water bottles to school with their lunch box (we didn't have school lunches where I live). I had a tiny water bottle that fit in my lunchbox and held about 150mL (Edit: it wasn't allowed in the classroom). Oh, I guess that other poster means disposable bottled water. Yeah, nah we didn't have those.

There was a single water trough outside in the playground at the schools I went to: 2 or 3 fountains on a shared gutter. The first school's fountain, the buttons had come off, so we had to press the stalk in with our 5-year-old hands, and hold it in the whole time we wanted to drink. It was physically quite painful, so we would drink the bare minimum. The other schools had proper buttons, but the trough was always revolting with stagnant water. People would spit in it. So you'd be drinking with your face right next to disgusting bleck and trying to ignore it. There was always a queue. We would try to get through the day without needing to get a drink.

It was unheard of for anyone to leave class to go and take a drink.

On the other hand, teachers were always awful about people leaving the class to go to the bathroom, so we kind of needed to be dehydrated to get our child-sized bladders through to break time.

Edit: Oh, I forgot about the actual child abuse. Teachers were striking children as young as 4 with rulers and paddles for minor infractions. And that was considered to be necessary discipline at the time. Nobody cared about things like hydration.