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. The only exception to these timeframes is when you are conducting school activities outside of the school building, such as a class trip. If a teacher returns from a trip during his or her scheduled lunch time, he or she is entitled to a duty-free lunch.