Naisho no Jikan

Twenty-three-year-old Daisuke Aoki has just landed his first teaching job as a grade school instructor at Futatsubashi Elementary School. He is placed in charge of Class 3-1, where one of his students, a mischievously precocious nine-year-old girl by the name of Rin Kokonoe, develops a crush on him and goes so far as to proclaim herself Aoki's girlfriend. She aggressively pursues her efforts to be with her teacher despite the fact that he will lose his job if she gets too close, a situation further complicated by the often complex, intertwining relationships existing between them and their respective friends, families, and peers.

[Edited from Wikipedia]