as she would no doubt be the person they would contact in a case of truancy or absence from school, then I can understand why they'd want her to sign an application for approved absence tbh. I don't consider, IMO, that this is in any way discrimination - you may have parental rights (the correct term I think is parental responsibility), but she has responsibility for him on a day to day basis, and in relation to him attending school. Do you think she'll object to signing the form or something?