Its commonly accepted practice that children are not considered developed, learned, or experienced enough to make life-altering decisions. Society and government have demonstrated this on controls down to things as common as driving, voting, alcohol, viewing adult media, tattooing, or piercing, etc...
Yet we debate forcing society and government bureaucracies to attend to the whims of a child who may “feel” they are not the sex they were born with. It is utterly illogical and absurd.
How to treat this child? We can ask - do you allow degrees of bullying based on other criteria? Why would you allow mistreatment of any child then?
Very simply, you hold them to the same standards as any other child, including their biological sex. They can “choose” their sex when they are adults.
Also, for your teacher’s own future legal liability and safety, why would you want to create any complex system, of unworkable and completely impracticable rules beyond this?