Just a quick comment on what I saw as a missed opportunity on the part of the students. The target was swinging in circles while hanging from a tree. No matter how fast he went, the circles were at pretty much a constant circumference. The teacher himself is a certain width, also constant. Had the students lined up with their bb guns as wide at the target's diameter, and had they been closer together than he is wide, there should have been no problem hitting him if they all fired in unison.
Now one could argue that it's impossible to think that they could all shoot at exactly the same time, giving the target a distinct advantage. But I think if they continued firing (as close as possible) in unison, I feel that at least one of them could still have hit the target.
Now this is only episode two. So obviously they aren't going to kill him. Unless they have different targets throughout the series (I haven't seen more than these first two episodes). But, I'm guessing, if that's the plan for the series, I see one target per season making the most sense.
Otherwise a good episode. It's hard not to like the teacher and root for the students at the same time. He's a villain that wants to destroy the Earth, yet he's clearly the best instructor these students could hope for, especially considering they're a group of misfits looked down upon by the rest of the school - students and staff alike.