An interesting paper (The 2005 Sodexho School Meals and Lifestyle survey ) that states a lot of interesting facts on hydration of children in classes, the PDF is based on 2005 statistics, but thought it valid for the discussion.
http://www.montgomeryspring.co.uk/pdf-downloads/Drinking-in-Schools.pdf
Taken from the PDF -
"For those schools that do allow drinking in the classroom, many types of drinks are taboo. Thirty five per cent of schools discourage drinking fruit juice, half do not like pupils drinking any kind of squash and most discouraged any kind of carbonated drink – even sugar-free and caffeine-free variants. We acknowledge that there are sometimes practical reasons for this – risk of spillage, staining, sticky fingers, etc. It does need to be recognised, however, that limiting the types of drinks consumed in the classroom may have a negative effect on how much children drink during the day and, therefore, on hydration levels.
For instance, the suggestion made in some quarters to ban specific foods and drinks from being brought into schools would seem to be impractical and inappropriate. It would appear that the reasons behind the proposal are to control children’s dietary intakes, but rather than banning foods or drinks, surely the key to compliance and better dietary habits is education? Already, lunch boxes are fairly healthy on the hydration-front and, according to last year’s School Lunch Box Survey from the British Dietetic Association, 76 per cent of lunch boxes already contain water, squash, milk or fruit juice.
There are talks about removing vending machines from schools altogether, but that would be an over-reaction. It would be better to use them as a positive vehicle for enabling adequate hydration. Perhaps a selection of bottled water, dairy drinks, low-cal drinks and fruit juice might provide children with a sufficient choice of ‘healthy’ beverages that would encourage them to drink. Getting children to drink more frequently helps them to stay hydrated and water or milk are tooth-friendly for sipping between meals."
I don't have children myself, but i would have thought that a healthy balance is needed to keep kids hydrated through the day, if that means with a little added zing (squash) to the water to make it taste better (low-sugar) then i don't see the issue with this.
OP - Speak to the head of the school, write a note or ask them to visit you at home, so you can explain your side as a parent.