How academy and mentee bonus xp works?

As the title stands, I am asking this question for two reasons:

after level 60 my mentee xp/sp bonus is gone randomly, sometimes I have it sometimes I don't.

and second when I get my academy circlet it work fine for a couple of hours, but then I have no item skill nor buff when using the circlet...


I am quite newbie, and dunno what I am doing wrong...

How academy and mentee bonus xp works?