You could change schools. Seriously. I had a year with 3 kids in elementary school and a baby at home. From the beginning of the school year till we moved in November, I had a total of 5 days that I didn't have one or more of my schoolkids home sick. Then we moved. Different school. They stopped being sick. I know of a school that shut down for a week because so many kids were staying sick. It really could be the school.
Another option, have you considered homeschooling? It's amazing how much healthier the kids can be. It's an option that more and more of us are doing for various reasons. We are in a group with a great support system, lots of field trips, and bi-monthly "class days" where we get together with other families and the kids have classes together and get to play. We love it.
Hope this helps,
T. - homeschooling mom of 4, including 3 great teenagers!