Kind of like a shorter, cuter version of GTO, Seishun Shiteru Kai follows a young and unconventional teacher as she deals with a smart student who is out to cause trouble in school.
Most of the story revolves around the young teacher trying to demonstrate to a smart, popular, yet troublesome student that teachers can be trusted. She bails him out of trouble, often landing herself in the same trouble, and (this being a shoujo romance), eventually the two start to have feelings for each other.
That being said, this is a fundamentally silly manga. The situations they get in are mostly absurd, and are
often resolved with the help of a pair of young kangaroos. The art-style, which makes everyone look even younger than they actually are, plays into that sense of whimsy.
All in all, if you like silly humor and teacher-student romances, this is a very enjoyable manga.