Can entities (spirits, faeries, angels, etc.) act as teachers or trainers to characters?
For example, could the spirit of a magus teach arcane abilites, arts, or spells? Could faeries? Could angels teach appropriate abilities (theology, divine lore, etc.)?
If so, wouldn't some covenants summon and bind or compel some entities to teach?
Some faeries can teach, but this is rare. The faeries who can do it have usually stolen the skill from another human, and their training methods are heroic and impractical, often involving montages.
Spirits cna teach, and yes, the Cave of Twisting Shadowa, for example uses its ghostly residents in lieu of a library.
Some Angels can teach quite well! (Ars Notoria)
At least some Demons can teach, but this usually causes the Corrupted Ability flaw.
Most Magical Beings can teach (ghosts may be an exception).
Most Faeries cannot teach (don't have abilities per se).
very cool, thanks. This should lend a bit of flavor to my home saga. 
Yes. In my last saga, our spirit master spent five solid years in research, spell development and the like, before unveiling to his fellow covenant members a library comprising the bound spirits of a load of dead Diedne (It was a high-Diedne-quotient saga) and a Danish rune-mage. It was very well received by all save the christian wanna-be-a-Templar, who left the covenant. Good times.
I just suggested a similar thing in the berserklist for an enchanted forest. Bind spirits/ghosts that teach stuff sound like something that should be far from uncommon in ME.
Cheers,
Xavi
Can ghost magi still teach the arts, spells and the like?