She says that her union has advised that this is discriminatory. Is it?
IT might be, but only an employment tribunal could decide.
It is now well established that a good employer should allow mothers to return to work part-time and that not to do so would be discriminatory, unless the employer can demonstrate that the job can only be done a full-time basis.
In schools, this often creates difficulties, because of the need to provide continuity of teaching, leadership an pastoral care and each case has to be treated on its merits. However, the teacher is in no position to dictate her own terms and the best outcomes are likely to be the result of negotiation.
In this case, the presumption has to be that she returns part-time to her whole job, not just part of it. Her salary will, of course, be proportionately reduced and this may free up enough to fund an assistant head of year, who will work in partnership with her.
This does not have to be the same person who is sharing the teaching timetable and could be an opportunity for someone else to gain valuable career experience. This may not be an ideal arrangement, but it will avoid the charge of discrimination.