Tell them to provide this to you in writing - then take that to an employment attorney for advice. Since this has been your schedule for over 2 years, and they hired you already knowing about your teaching commitments, they can't suddenly pull out the rug.

I had an employer try something similar on me - I recorded the conversation with HR, and then let corporate know and the HR person was escorted from the building shortly thereafter (and I was never bothered again).