The problem with the fillers is that they derail certain characters at times. This time the most obvious was once again Yukio, but Mephisto also acted weird and inconsequential. First this guy gets all serious, and after a really random intervention from the Vatican that he could have stopped easily, he just walks away, abandoning his students that are held hostage. Make up your mind, will ya?
But yeah, Yukio: I still am not buying his subplot. It’s annoying and just feels forced. Like the Konekomaru arc, he’s overreacting way too much and just keeps on angsting about having to protect his brother. That’s just not interesting and gets annoying really fast. It’s not the kind of subplot you’d hold secret for the viewer, especially since they’re being so un-subtle about it.
The rest of this episode had the right spirit, though. Rin’s classmates have been handled quite well in this original story, and this episode solved the biggest barrier between him and them: Rin’s flames. He may have learned to control them a bit too sudden (isn’t it more logical if you gain control of flames like that gradually over time?) and that teacher also was surprisingly interesting. Again pulling his wife in was a bit cliched, but there definitely is more potential in him now than Satan.
Also, why does Yukio shoot with tranquilizers? I mean, with Rin I can understand and all, but against monsters who threaten the lives of his friends. Dude, get a bazooka or something. Anything better than those “it may look like I’m carrying a lethal weapon, but THOU SHALT NOT KILL, KIDS!”-phony weapons.