SOLVED. Teens/ YA Fantasy. Female lead is an apprentice of magic at a tower (she is the only student of the wizard). Her only friend is a boy only she can see, who is later revealed to be a ghost. [s]

[Jasmine]

I think the wizard turns out to be the antagonist and tries to steal the MCs magical powers and she had to leave the tower and her ghost friend behind (he can't leave the tower) but I can't be sure

I read this as a tween about 15 years ago.

[Becca]

The Valley of the Wolves by Laura Gallego García

[Jasmine]

This is it! Thank you so much Becca. I have been looking for this book for years!