I've never done anything like that before, either, but I would definitely start by inviting alumni back. Some sort of "Homecoming" event would be appropriate. Especially if you can get any "distinguished" alumni to speak at your gathering.

Our Elementary school was erected 50 yrs. ago this school year. I am the Secretary of our PTO and was thinking of having a celebration of some kind. Wondered if anyone has done this type of thing before and looking for some ideas? I was thinking of making it a town celebration, with a huge cake, punch, balloons, games and some sort of display of the schools history, full of photos and information. Any input would be greatly appriciated