@
LeopoldBloom In what way can parents cause great harm to their children without violating the law? What is your idea of harm?
How do you define it? What is its extent?
Does it concern anyone else apart from the child's family?
"Potential harm" doesn't exist. You do not preempt harms in matters of family life, or anywhere else. It is a parent's duty to create an environment where their child feels safe, and able to discuss anything with them. However, because children are children, it can be difficult for them to discuss everything with their parents. This does not mean that the parent has failed in creating an environment where the child can confide in them.
It is not the school's concern how parents choose to raise their children, excluding criminal violations. As long as the teacher has not seen any possible criminal violations (example: physical neglect, physical abuse, sexual abuse), they should not be interested in the child's family life.
"Which may not rise to the level of criminality"
Do you realize how dangerous your threshold is? It means that parents who are not abusing their children can be treated as if they are, and as if this is an objective reality. This thinking is authoritarian.
If a child requests to be called a pronoun that is not reflective of their biological sex, a parent should be notified. If children are too young to consent to sex, to purchase alcohol, to get married, to vote, etc. they are also too young to decide, without the knowledge of their parents, that their gender does not reflect their sex. Your hypothetical is not comparable, as it does not concern self-identification on the basis of sex and gender, which are intrinsically related.
If this is a true case of gender dysphoria, it is probable that the parent will already know, or have some awareness, that their child is not presenting as their sex or the gender presumed to reflect their sex.
Most fundamentally, a teacher should not call this child by their new gender, without the knowledge of the parent. The relationship between the parent and child is natural and innate, and teachers do not need to interfere with it. Society does not need to interfere with it.
We are raising our children, not the state. Teachers are not self-appointed babysitters, nor are they a child's "friend". We pay them to teach our children, and nothing else.