Yeah the episode was awesome, can't wait for the next one.
Spoiler:
Yeah it could actually be Mouth, I mean he was there at the shooting, it's possible. Or what about that girl who was also at the shooting but who Jimmy let out early? Do we actually see her leave the school, because if not, she might have just hidden somewhere else in the school. And yeah, Derek could appear on the finale, I'm hoping he appears a bit earlier though, plus I'm not too sure that the producers would focus on that particular story on the finale, I'm guessing the finale will focus around something with Dan, Lucas, and maybe Deb and Karen, probably someone finding out that Dan killed Keith, or maybe even Dan telling Karen or someone.