Is it? I’m pretty sure there are considerably more nutrients in, say, orange juice?
Do you know how much sugar there is in orange juice? Even pure, freshly squeezed orange juice? If I were to replace all of my drinks throughout the day with orange juice, the comparative sugar attack, plaque build up, tooth decay with water would be obscene.
I'm a teacher. We have the same rule at school. I drink water throughout the day to practise what I preach. (I'm a shit host - when people come over, I can literally only offer them water or wine).
As PP have said, water is better both for children's teeth, and doesn't create a sticky mess when bottles inevitably leak.
I've had parents telling me for years that their children don't drink water. In all of my years of teaching, I've never failed to encourage these children to drink water and plenty of it. I'm sure there are children with severe SEN who genuinely can't be persuaded, but in my personal experience, children who are thirsty will drink what is on offer.
For me, it is a battle worth fighting. Dental work is so expensive and you only get one set of teeth. My dentist told me that it is the drinks we ingest that are the main culprit for tooth decay, and if I can make a small difference to the children that I teach in this way, then it's worth it.