My class has a formal reunion every 10 years - used to have them every 5 but people said "enough" - from what I hear, they also usually get together every few years and go to Homecoming - and nearly 45 years later, they still get trashed.
Our class was the biggest - 300+ and the reunions are usually full events - about 150+ people, though from what I've heard, there are a lot of "crashers" from the class before and after ours. Teachers as well, though most are gone now. My 20th was fun - I plan on going to the next one to see who all these FB people are - they've decided with so many crashers, it's going to be a combined year reunion, since I guess we're the only class that gets a good turnout. Funny thing is, when we were in school, we were kind of the loser class.