I don't think there is any sort of definitive answer to your question. There are a great deal of systems that are built primarily around spellcasting though, so I'll list a few ideas that might work here!
Mage the Awakening: This might be a bit too crunchy for your tastes, but it easily provides for a great deal of player agency in spell creation and the like. To better fit the school setting you could easily restrict player's access to the different levels of the Arcana as well. I'd recommend this over Mage the Ascension since that game has lots of extra baggage up to and including
consensual reality that would muck up a game about a magic school.
Cj Carella's Witchcraft: While Unisystem is rather obscure these days, Withcraft is a rather well put together game. It has quite a few different types of magic that allow for very different approaches to magic and the like. Admittedly the system is more of a modern fantasy one, but it could definitely work well.
Savage Worlds: A relatively popular and light system with fast combat and some rather slick rules. It has a power system that is pretty easy to understand, and with modifiers known as 'trapping's that allow players to personalize their spells. You could easily grant players new powers and perhaps change the amounts of power points available.
Freeform/Rules Lite: I could recommend a rather simple system such as
The Window or even to just have a very simple die rolling system. The main advantage of a freeform system in this sort of game is that you get to define how magic works. Do you want to have magic be somewhat lovecraftian with mysterious rules? Do you want magic to rely on deals with supernatural creatures? Do you want it to be a sort of mystic science? A freeform/rules-lite system gives you the most opportunity to play around with such things. Since your teachers will be giving lessons based on how magic works in the world, it may be best to follow such a system. Glad to discuss more ideas though! I love magical schools!
This message was last edited by the user at 21:16, Tue 12 Sept 2017.