GTO. Great Teacher Onizuka. This manga has EVERYTHING: Action, comedy, suspense, drama/romance, and flat out lunacy! And best of all, none of it ever seems contrieved, over-the-top or out of place - this wild mix of elements is not only blended together perfectly, but in such a way that it actually feels like something that could happen in real life!
The hero is Eikichi Onizuka, a lazy, unambitious punk who suddenly decides to become the greatest teacher ever. He is eventually able to get a job at a private academy, where he is assigned a class so rebellious that teaching them is considered a suicide mission. (They actually make several attempts at his life!) However, as Onizuka remains more of a street punk than a teacher, he is able to relate to the kids' problems and goes to great lenghts to help solve them.
The manga deals with everyday situations as well as more alarming problems, and much of the fun comes from Onizuka's nearly superhuman ability to come up with the most insane solution to any problem that may appear.
(A somewhat tame example, so I won't give away any of the REALLY good stuff: One girl is upset because her parents are getting a divorce. Onizuka attempts to remedy this by showing up unannounced at their house one evening, shirtless and carrying a sledgehammer, and whacking a large hole in the wall separating the parents' bedrooms, forcing them to share each other's privacy.)
Bottom line: Onizuka is a truly great character, and GTO is one of the best comics out there. I highly recommend you try it!