Apps Gone Free is an app for the iPad which gives a list of 3 to 8 paid apps that are free that day or free for a short period. Today (10/5) Math Blast is free. I've never used this particular app but it looks like it would be nice for elementary students. I found out about sandbox the other day due to this app and they've sometimes featured high school math apps. They often feature photography apps, presentation apps or art apps, all of which could be incorporated into the classroom for use in projects.
The app itself is free and well worth having. The list of apps do not appear before about Noon EST so I check out the list when I get home after work. If your school is under budget restrictions, this may be a way to get some paid apps you've wanted.