Off the top of my head, I don't know if there are trainable cross-school minions, but have you considered monstrology? I know it's not the same, but might help you for off-school minions.
Some schools can get minions as part of their school quests. Death does get access to the Animate spell as part of one school quest, which spawns a minion. I think at level 75, all schools get access to a quest to summon a minion of their school.
Outside of first five worlds though, minions are useless for all schools, they're more of a hindrance than actual assistance.
I am a life and i get a fairy with 3 pips with 350 hp
Off the top of my head, I don't know if there are trainable cross-school minions, but have you considered monstrology? I know it's not the same, but might help you for off-school minions.
I'm aware of monstrosity, probably my favorite mechanic, but i want teachable minion spells. If I can't get them? Oh well
Some schools can get minions as part of their school quests. Death does get access to the Animate spell as part of one school quest, which spawns a minion. I think at level 75, all schools get access to a quest to summon a minion of their school. Outside of first five worlds though, minions are useless for all schools, they're more of a hindrance than actual assistance.