What's it about?
Nerdy teacher really is a yakuza princess and helps her loser class by kicking ass. In three series. And a movie.
What do I say?
This actually is a must-watch, because it's so famous. Even if is totally formulaic: delinquent gets into trouble, Yankumi takes off glasses, beats down army of thugs, delinquent is instantly reformed. But nerdy Yankumi is cute and it basically stars more Johnny's than you can shake a stick at (if you like that sort of thing and why wouldn't you).
The first Gokusen is the best in that regard, as it stars MatsuJun as the main delinquent. MatsuJun, like he always does with the ladies, somehow manages to have oodles of chemistry with Yankumi. That led to a romance plot being tacked onto the end of the show - in my opinion hastily rewritten after seeing the chemistry.
Alas, the plot never got anywhere as the Arashi train left the station right around that point and MatsuJun was swapped for Kamenashi Kazuya and Akanishi Jin in season 2. Who never have any chemistry at all with anybody. (Well, I guess that depends on how literal you want to get with 'chemistry', I guess, judging from the pictures that are circulating over the net). But anyhow, even so season 2 was kind of cute, too. Season 3 got somehow repetitive.
So go ahead and watch Season 1, and then, if you like the plot you can watch the others, too, as there will not be much change. Even so, I recommend fast forwarding every time you see Yankumi's father appearing.