I reckon Merle Ambrose is Myth Reason 1: His wand is for myth school only. When you first start w101 Merle Ambrose gives you troll as the first spell you use (I think, pls correct me if I am wrong) against malistair's minions.
Reason 2: He looks like a myth wiz compared to any other school. Plus his pet "Gamma" is a myth pet. When you go into the room Gamma is in you see all myth things.
Conclusion: I'd just like to wrap it up, that is my opinion, tell me what your oppinion is.
Hunter, pryomancer, lvl 57
He said he trained in all schools but started in astral.
Well, he is the Headmaster of the school so I'd say he has mastered all of the schools. I mean, imagine if you fused all of the mastery amulets into one, that's him. Headmaster Ambrose isn't bound by our rules lol!