Sadly this is all too common - most teachers haver a contract stating either numner of days of percentage contract i.e. 0.5 Effectively this allows the HT to delegate their work as they see fit for the school. Often when a teacher goes part-time they negotiate a cetain day off - frequently either side of the weekend and anticpate this is written in stone - its not - soon as the HT sees that the school's needs have changed this can be changed..... If the HT is going for it all guns blazing - they maybe wishing to eradicate parttimers - for some they are the bane of their life..... Being constructive I would go and see the HT and offer suggestions that may work better for you, perhaps in collaboration with a colleague who you share a subject with.... it is always better to go with solutions to problems than solely the problem.