Learn to say NO:)) Sorry, I'm a mom of 3 and get overwhelmed with all the things. One thing I learned about the fundraisers, is that the school is just needing money for something. A few years ago, I asked if I could just write a check instead of selling things that no one wants or needs. They thought that was a great idea, and now that's an option instead of fundraising. So, that saves a lot of time.
As for birthday paties, the best piece of advice is to shop the sales for gifts. When you find something great on sale, stock up! Stash it in a box in the basement, that saves a last minute runs to the store. Let your son decide which parties to go to. My kids sometimes choose not to go. It seems overwhelming at the beginning of the year, but will settle down.
Buy a huge calendar-I have a desk calendar stuck on my back door. That way I can write down when everything is, when things are due, special days at school-you name it, it's all there.
I also have a filing folder for each of my kids for all those papers my kids bring home. As soon as they come home from school we sort through-if I don't need it, I throw it away immediately (that takes care of LOTS of it:)) If I can sign it and send it back, i do that right away, then I don't lose it. Very few papers actually go in the folder for later, because I deal with it all right away-makes it a little less overwhelming.
As for helping in the classroom, there's lots you can do at home. This is the first year all of mine are in school, so I haven't been able to help at school much until this year, but I've been able to help a lot. I've cut and colored things for teachers, organized scholastic book orders, counted Box Tops and Campbell's labels, and sent in lots of items for activities at school-all from the comfort of my family room. Just don't over-commit. I did that my first year, and really regretted it. Just slowly ease yourself into all the activities.