Madison Teachers, Inc. Solidarity Newsletter, via a kind Jeannie Kamholtz email (PDF):

MTI’s Teacher Collective Bargaining Agreement provides that all members of MTI’s teacher bargaining unit will be provided with a daily duty-free lunch period of at least 30 continuous minutes. The 30 minutes cannot be abridged by one being directed to walk with students to the lunchroom.
More recently, once again, some teachers have been requested to open their classroom so students can have “a place to go”. Directing a teacher to sacrifice any portion of their 30 minute duty-free lunch period violates the Contract. If a teacher volunteers to do so, they are to be compensated at $9.10 per hour, with such computed in one-half hour lots.