As you said yourself, it was never explicitly mentioned if he was still in the Air Conditioning Repair School or not.
Seeing that he has left the show by now, this secret probably will never be solved completely.
However, there are some strong indicators that he left the AC school at the end of Season 3:
- Apparently he was already finished with the two year program after his short time there. In S04E13 he says:
Troy: I passed all the classes, so now I just do yoga while the teachers write down my wisdoms.
He took part in a "normal" Greendale College class in Season 4, titled "Nicolas Cage: Good or Bad?" So it appears that he is back in the usual Greendale course program.
After Pearce's death, he embarks in a trip around the world to get the inheritance. So if he had not left the school before that point, he definitly did then.