From Shinmakoku Scanlations:

When Tanami Riku's father died in a traffic accident, he was left all alone. Then suddenly a guy named Onikubo Nagi came and demanded to be his guardian in place of his father, who was actually Nagi's former teacher. Then he applied as Riku's homeroom teacher! He pushed himself into Riku's life without even asking for permission. Living with the Super Sadistic Teacher was unbelievably harsh. What exactly did Nagi want with Riku?