You are entitled to a duty-free lunch period. This was a strike issue in the 1960s. Duty-free means just that. It’s free of any and all duty assignments.
If you teach in an elementary school, you must have a daily 50-minute duty-free lunch period. In a secondary school, you must receive a full period for duty-free lunch each day.
You can see what the contract says about lunch periods by reading articles 7A4, 7B6 and 7C4a.