My high school class never did a 5th reunion... and the 10th won't be for another 2 years... so we'll see if they do that. I can't imagine who would organize it.
My college has some sort of reunions during homecoming... maybe every 10 years? I've definitely seen them for 25 and 50 years. Maybe the individual schools within the college have their own - I'd definitely go to one of those.
The school I'm at now has a reunion weekend during the summer, every 5 year mark has a reunion, and they even house them on campus! They have all sorts of parties and free food and drinks and whatnot. (Ah, to have that Ivy League money...) It's an insane jumble of shuttle buses, and families wandering around looking lost, and next year I plan to stay away as much as possible that weekend.