Against this proposal, for mutual respect
As a parent, I find this completely unacceptable. The psychological challenges that accompany transgender identification require family support. If a child chooses to identify this way, it is not for the school to withhold this information from the parent. Honestly, it's not a small decision, so I can't imagine parents wouldn't already know? Isn't it in the best interest of the child if they can work with teachers and administration to properly support their children?
As far as the bathroom issue, a third, non-gendered, lockable individual bathroom and locker room should suffice. You are talking about a very small portion of the population at each school. Leave the boys' bathroom for boys born and identifying as boys and the girls' bathroom for girls born and identifying as girls.
If this is a proposal advocating equal rights, and respect for all, then respect the parents of these minor children and the personal space of the cisgender students.