Ansatsu Kyoushitsu also known as Assassination classroom, It has 22 episodes It is must watch anime series It has unique story Tentacle monster comes out of nowhere after blasting half the mooon to class 3E to become their home teacher and gives them a goal KILLING their own teacher till end of their session or he will destroy the earth just like moon they named the monster Koro-sensei the students in class 3E are all there due to lack of ability in sports or academic region, The story revolves around student discovering the past of koro sensei and re discovering themselves.
It has a good blend of action and comedy . (for more information http://myanimelist.net/anime/24833/Ansatsu_Kyoushitsu_TV#) It has interesting set of characters
I would suggest reading manga over anime as it elaborates story in a more better way than in anime