He's not really studying with them. He's more just part of the "group." They study together, he's ehh.. there. It's a little bit weird but it's the best we can get. In this episode he was just attending the farewell party but, yeah, he's still a teacher. After the Ass-Crack Bandit episode Community had to devote two…

I thought it was just Troy and Abed who admitted to being dead? I mean technically every character but Britta touched the lava but only Troy and Abed embraced the fact that they were clones now.

I think Season 4 was subtly titled "the gas leak year" by one of the study group members because it was so crazily off.