Assassination Classroom by Yusei Matsui

Class 3-E is loser class.  The students have all been relegated there by their bad grades.  Their school life all takes place in a rundown building nearly a mile away from the main school.  They're looked down on by all the other students and teachers.

Then one day, they get a new teacher.  He appears to be an octopus-like monster.  They have one year to kill this monster before he destroys all of earth.

Can the students finally excel at their studies enough to kill the teacher and live to see another school year?

This is a wonderfully funny manga.  Although the topic of assassination figures into nearly every page of the story, it isn't gory or gratuitous.  The students are interesting and the teacher is fascinating.  A great series for teens.