When I was in elementary school, we didn't have drinks available except at lunch time or when passing a water fountain; we certainly didn't have water bottles at our desks to slake our thirst any time we wanted - drinking in between regular breaks simply wasn't needed by healthy youngsters.
The story said it was one of the hottest days of the year - 28. Presumably centigrade, that's only around 82 F, which is warm, but not exactly hot. Open windows were how we dealt with somewhat higher levels of "heat" back then.
But if the school practices said otherwise, and the teacher denied a student water because of a Moslem student's fasting . . . she in effect imposed Moslem religious observances on other students - which ought to be a firing offence.