I'm not averse to the 'only water' schools - and I had a water refuser. I understand the rationale behind it and would support it for all those children without a recognised need.
My DD was hospitalised twice for refusing to drink. She went from breast to suddenly nothing - wouldn't take formula, wouldn't take milk, wouldn't take soy milk, wouldn't take water and so on. This was at 8 months. Doctors suggested smoothies and yoghurt drinks - that worked. She had watered down smoothies for the next few years (Sainsbury's strawberry and banana - no others)
At 3 we moved to a hot country and the pre-school was water or nothing. We listened to everyone who said "she won't die, she'll drink water once she's thirsty. Well she didn't and I'll beat myself up for her second hospitalisation for the rest of my life. She was really fucking ill. The doctor tore me to pieces, and when he'd finished with me her tore the pre-school too. They allowed juice and squash from then on.
She drank 50/50 apple juice and water until she was 8. Then she started drinking water just as suddenly as when she stopped.
We are in UK and no child here will die if they drink squash/juice/whatever at home but nothing at all from 9-3. They may dehydrate on a very hot day, but not to dangerous levels. We ensured that DD had a very large drink before school, access to water just in case (never drank) and DH met her at 3 with another large drink that was generally finished before he was even out of the car park. There is very little need for juice drinks during the school day if you have a child who won't drink water - parents just need to make sure they drink outside of school.