All other lightning spells belong to pyromancy, so I think ball lightning should be, too.
At least the description must be updated, since evokers are not using fireballs:
Master evokers, bored of simple fireballs, started toying with the control of balls of lightning. ...
I've got a bunch of flavour text changes awaiting review in the
spelldescbranch that fix this description (along with many others).For the broader question of "what schools of magic should the different spells be in", I have Opinions
I think the discussion of spell attacktypes, lore summaries, and spell schools is a never ending and and always evolving discussion.
So, a spell is modified. IMO, as long as there is an explanation or reason - then it's all good. But, I also feel there should be some in game lore to explain why a player may suddenly lose access to a spell.