I really like that idea of secret trainers in Pirate101 and would wonder if for some of the lower level spells, you can carry that over for any wizard to get?
Not school specific spells or any high level spell, but some of the 4/5 pip or less spells that are only available via treasure cards.
You could even have it like you did in Pirate101 where you can only get them from one place during certain times. I think in Pirate they rotated every 8 hours was it? That way even people in different time zones still had the opportunity to visit the special trainer.