I like "you can't teach spells"; also interesting would be to have the teaching of spells be a separate move requiring experience, and have it be risky as opposed to an automatic success. A spell, here, isn't a rote recipe or algorithm, it's a way of interacting with a move. Teaching someone to cast a spell is a separate skill from casting a spell, and it requires that you actually cast it while they're with you, and that they participate, and their participation is risky and can end in disaster.
So perhaps you have to take the advance that allows you to teach magic, and then to teach a spell, you cast the spell (requiring its own roll, risk, and prerequisite situation) and you also roll+something to see how well you did on teaching -- maybe you roll+relationship (hx/bonds/etc), mirroring the traditional intimacy of master/student relationships -- and a miss there counts too...