Is it possible to train a character in teaching while yourself teaching a group? If so does the trainee have to have the skill you are teaching?
I would say unconventional but there is nothing in the rules to forbid it.
In fact I would say it would be a wise use of time and a appropriately medieval thing to have a younger magus watch and learn while a Master teaches an apprentice.
Or student teachers at university.
A book on teaching might be more efficient but the story of training the skill, much much better.
There's nothing forbidding it in the rules. If you're wondering how people pick up the Teaching skill - Art & Academe gives rules for picking up xp in Teaching while studying Latin or Artes Liberales and suggests this is how most teachers in the 13th century pick it up at university or cathedral school.