Is there a way to turn off wireless access in a classroom?

It’s not possible — access point deployment is such that one AP could serve multiple classrooms and offices; furthermore, the nature of LWAPP/CAPWAP technology is such that all APs are aware of the entire system of APs within their coverage area. If one AP is turned off or dies, all the other APs in the adjoining area increase their signal strength to fill in the coverage gap.

For a look at what other universities are recommending to deal with this issue, see http://carleton.ca/edc/wp-content/uploads/Wireless-in-the-classrom.pdf