Taken from Matthew's Anime Blog:
Daisuke is a 23 year-old new schoolteacher. He ends up being put in charge of a 3rd grade elementary school class since the previous teacher quit, and in that class there's a girl who's quite a troublemaker, Rin Kokonoe. You could call her a masegaki (used to describe a kid that acts more like an adult in some ways), and she's also part tsundere (though more dere than tsun).