Magic or Education?

Just for one bonus? Have you tried taking Secret Knowledge + Volatile Mana? The effect is the same, but you get more spells for the Spiritual Bond.

And is it even necessary? The bonus to levels and characteristics only matters at high levels, where there's something to add. Necromancy knowledge is needed, yes, but with Sorcery and Artifacts, you have enough mana for spells. So, I think education isn't necessary for a Necromancer, only in the late game, and even then, Zoltan has it built-in and it has a direct impact on his ability.