There are 2 issues here. Are these 13.5 hours included in the school's directed time? no teacher can be directed to work at the weekend by the way. Should a teacher fail to follow a reasonable instruction as part of directed time the headteacher could formally discipline the teacher but there is nothing in the PM regulations about such an action impacting on PM. I would contact ther local union office(s) and obtain advice from local officers. Head sounds like a bit of a control freak.