Well, this is a new mangá series in Shonen Jump.
I started to read it and its kinda fun, strange story, but fun

"The students of class 3-E have a mission: kill their teacher before graduation. He has already destroyed the moon, and has promised to destroy the Earth if he can not be killed within a year. But how can this class of misfits kill a tentacled monster, capable of reaching Mach 20 speed, who may be the best teacher any of them have ever had?"

http://www.mangaupdates.com/series.html?id=80627

9 chapters are out at the moment:
http://www.mangareader.net/assassination-classroom

assassination-classroom-3424455.jpgAssassination-Classroom-1-627x320.pngassassination-classroom-3456649.jpgassassination-classroom-3424535.jpg