The Power of Z!
I think the whole "graduation" concept is just a red herring at this point. It was mainly used to attract kids who are used to slice-of-life shows. I think what is actually going to happen is Ash will be attempting all the trials in order to get a z-crystal, but along the way gets another goal (fingers crossed it has to do with Aether Foundation and whatnot), and Ash will be doing some other stuff alongside trying to master z-moves. The school will be used for fillers most likely, which I am perfectly fine with. I would much prefer new interactions and insight on recurring characters rather than random CotD introduced only to never be seen again.
Now that Ash has already decided to participate in the Rite of Island Challenge, about 75% of me is inclined to agree with you. The graduation concept was probably part of the marketing effort to attract viewers