No matter how important the clay is to the kids (and as a middle school teacher of 26 years I know it is) you are doing your self no favors in the health department. I taught in the situation for a number of years. I finally had a friend who is a mass spectromitor (measdures gasses, etc.) come in and do a test for me. He was appalled at the air quality. He said OSHA would close down the room immedialtely if this was a factory. I told my principal that we would have to give up clay unless I had a new kiln that was properly vented. We did no clay for a few years until the money could be found.
I have really, really bad allergies that my doctor blames on my room-althought it can't be proved. I also had a benign brain tumor. Again nothing can be proved but under no curcumstances would I fire in those conditions. There are lots and lots of things you can do in a good art program without putting yourself, your colleagues and your students at risk. mlh