he has a male body but a female brain,
That is sexist
But if they want to play that idelogical game get the school to list what makes a boy brain different to a girl brain.
Even then that is a faith based belief as the child in question has not been tested and "found" to have a girl brain.
I would be looking at what type of gender woo the school is teaching as fact if they are going with girl brain
The school can have a school ethos but can not force your child to adopt the belief system of girl brain. And there was legislation passed 2015ish??? about schools accepting local students and adjusting ethos to not punish "non-believers" which was mainly aimed at RC schools
They have an obligation to respect your right to educate your child with your beliefs and not to try teach your child that your belief is "wrong".
THE FAMILY
ARTICLE 41
1 1° The State recognises the Family as the natural primary and fundamental unit group of Society, and as a moral institution possessing inalienable and imprescriptible rights, antecedent and superior to all positive law.
2° The State, therefore, guarantees to protect the Family in its constitution and authority, as the necessary basis of social order and as indispensable to the welfare of the Nation and the State.
EDUCATION
ARTICLE 42
1 The State acknowledges that the primary and natural educator of the child is the Family and guarantees to respect the inalienable right and duty of parents to provide, according to their means, for the religious and moral, intellectual, physical and social education of their children.
2 Parents shall be free to provide this education in their homes or in private schools or in schools recognised or established by the State.
3 1° The State shall not oblige parents in violation of their conscience and lawful preference to send their children to schools established by the State, or to any particular type of school designated by the State.
2° The State shall, however, as guardian of the common good, require in view of actual conditions that the children receive a certain minimum education, moral, intellectual and social.
4 The State shall provide for free primary education and shall endeavour to supplement and give reasonable aid to private and corporate educational initiative, and, when the public good requires it, provide other educational facilities or institutions with due regard, however, for the rights of parents, especially in the matter of religious and moral formation.
You have a fundemental objection to this ethos so you need to understand the schools policy and how the school plan to manage
a) "trans children"
b) children who dont believe in "trans children"
The school need to respect and manage both as they accept children of all faiths or none.
Imo the issue is the two children dont get on.
I would be working with the school to separate them as much as possible. And teaching your son not to bring up the issue as a weapon to be used against the other child. He needs to understand that doing that is bullying. So the context of the remarks are important.
If its a small ET school many of the parents will not want to become involved. But you need to look at who is on the BOM and how TRA they may be.
It will be a fine line so I would be pushing back on how the school is going to respect your childs right not to accept "girl brain".