One possibility is not to think of practice sessions as hour-blocks. If you have a piano available in school, you can do 5 – 10 mins. practice blocks whenever the opportunity arises. For this to work you will have to be very organised and consistent.
For instance, if you are learning a new piece, and just pick up a couple of bars and work on them for 5 – 10 minutes, after one week of doing this every day, they should be perfect.
Now comes the difficult bit. You must list everything you want to learn/master and divide in 5 – 10 mins. practice sessions. Then you must organise the several 5 – 10 mins. sessions available during the day so that everything is covered everyday. I think you get the idea. You may have to work in very little sections, but if you are consistent, it will add up. You can then use your one hour section to work on joining it all together.
Depending on how many 5- 10 mins. sessions you can manage everyday, you may have no choice but to limit what you are trying to accomplish. In any case it is better to have two or three items perfect, than twenty that are going nowhere.
If you do not have a piano available in school, you can still do practice away from the piano – Glenn Gould claimed he did all his practice this way. Another pianist renowned for learning and practising pieces exclusively from the score was Walter Gieseking, He actually wrote a book explaining how to do it (Walter Gieseking & Karl Leimer – Piano Technique – Dover). The idea was that you should have your piece completely memorised before you even got near a piano.
Finally, if none of this is feasible, you will have to drop some other activity to make way for practice, or sleep less hours!
Now, that I come to think of it, drop school!

Best wishes,
Bernhard.