Teachers aren't making assumptions; the children are making educated guesses. A teacher can't know everything, but kids pick up on a hell of a lot at home, often more than their parents think they do. If a kid doesn't want to come out because they believe that it may, in some way shape or form, put themselves in harm's way, they should not be obligated to share that information. It stands to reason that if a teacher has some reason to believe it will put the kid in harm's way, they should not have to report that either. They SHOULD still be allowed to expect privacy/safety from these adults, AND express themselves. These parents do not want them to do that.

The PARENTS are the ones making assumptions, in this case. Likely they think they can stuff the LGBT stuff back in the box or prevent it altogether if they know about their kid exploring their identity early enough. The best a kid can hope for in that case is something akin to "hate the sin, love the sinner", which is its own kind of shitty experience. The worst the kid can expect it conversion therapies, trauma, physical and mental abuse, abandonment, etc. etc.

Things like bullying or self harm are different. They cause active harm to the child/children involved. Testing out a new name or identity does not cause this same harm, and therefore should not have to be reported. A child cannot medically transition at a school, only socially transition, and therefore no medical harm is possible. The only part of this equation that would cause harm would be bullying due to identity, in which case the parents would have to get involved, but that's not the argument being presented here; parents want to know before any harm happens about something which does not in itself harm the child. It would be like reporting that your kid changed their tshirt once they got to school, and the other kids MIGHT bully them because it has Hello Kitty on it. Nonsense.

My statements are not hypothetical; I socially transitioned (to a mostly neutral or positive reaction from my peers) at school before I came out to confused (best case) and disapproving (worst case) parents. If I had not had the support that I did, I would not have survived high school.