Overwhelming, it is -- just get it all sorted out on paper and that may relieve some of that weight on your shoulders. Your topics seem appropriate, so write those down and designate how many pages you think they should take up. I would say you'll be closer to 80 pages before you know it! Don't forget about the advertisements taking up pages, too.
Our school here is very small like yours (about 40 kids per class); we don't have a football team because of the size of the student body. But, if your school has any sports teams, don't forget to include them in that. I know our school plays basketball starting in 2nd grade! Also, if there are any honor societies or clubs/organizations, you'll want to pay tribute to those.
Perhaps if you're still trying to find content for filler, consider a page listing student awards, such as 'students of the month' or perfect attendance recognition? And, what about field trips? You could fill space with documentation of those -- or classroom party pictures.
Did your PTO have a carnival or any PTO events that you have photos from? Those could be in a "Looking Back" section that could have the captions denote the event & ID the kids in the pic.
I hope you find these suggestions helpful!!!