Usually I do not bother myself with writing negative review (this is actually a common MAL trend as reviews are usually written by fans) as I have more or less good understanding of why tv in question are so popular despite being bad in my eyes. But this one is a total mystery for me.
It is pretty easy to split this tv in 3 independent "themes" and analyze them separately:
1. Class needs to kill their teacher in order to save the world.
Sounds fun huh? But it was not.
I expected a lot of cunning assassination schemes, clever traps, constant progress in methods
but alas - nothing of it. After few mediocre direct attempts to kill no one in class cares about it anymore, and story is shifted towards normal school live show + occasional pro-killer come to do the same dumb direct attempts to kill which is never gonna work by setting design (and therefore boring to watch). The lack of creativity is astonishing, I can't believe it was possible to do such a boring write-up with such potentially-reach material on hands.
Definitely a weakest part of the show. If you were primary interesting in this part - skip this show.
2. Class E is a class of problem children. School system is a bit evil. Classical story about making misfits into nice persons via solving their problems.
The show is struggle to find a right balance between assassinations and telling sad stories of the students. Both parts feels like incomplete attempts. But at least this part is done in more or less standard fashion. The bad part of it is the teacher character - he is very one-dimensional. He doesn't feel like a real person with any depth, and the main reason for it is a comedic relief. They wanted a comedy show similar to GTO, I assume. But while Onizuka provided plenty of fun while being a solid character which you can relate to, our teaches is just a clown with nothing to back him up. To make it worse variety of jokes are very limited and after 3-4 series you watched them all and they were not fun to begin with. This is huge letdown as show relies on it very heavily.
Score: If if take GTO as 10 than this show would be around 4/10.
3. Main plot aka who is Mr Teacher, why he needs to destroy earth, who is behind all of this etc.
Probably the best part of the show, unfortunately show is dragged into second season therefore there are no answers and overall amount of plot content is abysmal. The best way to describe this series would be - filler, slow paced, repetitive, quite boring and a bit absurdic. It really feels like you watch worst part of a good show.
Score: N/A, all in second season.
Conclusion: While I expect second season to be much better, I still believe it is not worth to struggle trough 22 episodes of filler material.
With 12 episodes format it would be so-so, but 22 puts it beyond salvation. And as it was released alongside with pretty solid tiles - I see no reason to pay any attention to it.