The fact that old spells will remain is not very significant. What is more important is where to get spells for the new school of magic? Presumably, nowhere. It follows that it makes sense to change the mentor's skills only to non-magical schools. The choice turns out to be not very diverse.
A lyrical digression about Greta's specialization. The Elven army is a strike force. If an Elf doesn't have the "ability to simply rush and attack" on the first turn, then it's not the right Elf. Because on the second turn, he will lose his army. Specializing in summoning a dragon is a burden for him. If he is summoned on the first turn, then 7 stacks will remain without a cast, and with "haste" and good morale (which is very good here), some stacks will already deal a couple of blows to the enemy. If summoned on the second turn, the dragon will constantly be late, and the remaining stacks will almost all deal damage without it. What is good for a Necromancer cannot be good for an Elf. Let's see what the defender of Moriton is planned to be like. So, skills: education (starting), summoning (summons a dragon, fills gaps in the fortress walls), mechanics (provides a witch), chaos (many artifacts for him, Daughter of Silanna - an energy channel, presence reduces mana costs. Greta only lacks mana if she actively uses "chaos"), defense (given that there are many Necromancers and it's necessary to stay in the castle). Which character's progression does this resemble, other than an Elf? In this structure, only "absolute defense" is missing.
The quests for which Karlam can get the most powerful spells are completed only in the very endgame, except for killing the merchant, but there the spells are more designed for dealing damage, so everything is fine. With the Tome of Power and Wisdom, you can learn all level 4 spells.