Weak Hero

The story is set in an average school where bullies rule and spend their days tormenting the frailest or the most-fragile looking kids in class. But then this one mysterious transfer student named Gray arrived and disturbed the usual set-up of the school. He may be smaller than the tyrants, but his fighting skills are way over the top that he can even make the biggest of all bullies lying on the floor and wincing in pain.