You have to train your Apprentices in one of the three disciplines: Alchemy, Arcanology, or Necromancy. You then train those students in one of the other disciplines, so everyone is trained in two, giving you Arcane Necromancers, Arcane Alchemists, and Alchemy Necromancers. You need those for research. You can tell by the little symbols that look like a flask with a star, or a star with a skull, etc.