I like the idea of school aquariums, they can be educational and also something to sit and watch and take the stresses away with.
But....
The logistics of keeping a school aquarium are massively complicated. As has already been said...weekends and school breaks. Not just the day to day feeding or the weekly maintenance to consider here. Feeding blocks are an absolute definite no-no due to their ability to totally screw up the water chemistry and kill fish as a result.
What about fish illness or disease...a fish won't get sick only during term time, they can get sick at any time and if there is no-one there to first notice the issue and then deal with it, you are going to have a disease wildfire that potentially wipes out the entire stock.
What about the filtration, heating, lighting....lights can go on timer (but in a power failure that timer will be all over the place), same with filtration and heating...powercuts, equipment failure there is so much that can go wrong and if you do not have a caretaker with 24/7 access, 365 days a year the potential for disaster is limitless.
In this day and age I cannot see any school granting unsupervised access to a school. Too many things can go wrong, the potential for trouble and the fish suffering is far too great.
If the school wants an aquarium then you absolutely must get a written plan done outlining all eventualities, sorting out full access maybe a suggestion of a parent going with you to maintain and check the aquarium might be the answer but I honestly cannot see a school granting you the priviledge of a set of keys without some sort of adult supervision. It would be a huge statement of trust by the school and I do not believe that any school would consider taking the risk of allowing a student to have keys, there is just far too much at stake and not just the fish.