Mithradites

You could mentor somebody who's self-teaching an evocation, though not directly teach them.

^ I knew there was something like that ruled. Was trying to remember the exact rule that was made.

What Sug said. At its core, evocation is the same in terms of connecting to the void and then summoning an element, but it branches off when you get to specific kinds of evocation. Knowing how to evoke ice and water is vastly different than knowing how to evoke fire. Because they both function differently and you have to take different things into account when you summon them. This is why someone who has mastered electrical evocation and can teach it could, in theory, teach a student up to the point of connecting to the void, but could not teach the student how to properly summon fire. The student would have to learn on their own at that point, and it would becoming self teaching.