The teacher wanted child's name and their interests/hobbies
The teacher asked the pupils to tell them "something about yourselves, just one fact" so, no mention of interest or hobbies.
As a young trans LGBT+ person this is probably the area of themselves they spend the most time thinking about and identify with, which is mostly likely why they chose that aspect. I would think this is more interesting / more of a fact than the girl that said "I like food and sleeping".
become friends through shared interests/hobbies as a person not an LGBT community member what like food and sleeping? LGBT+ issues can also be a shared interest if you're not part of that community. Even at a young age, teenagers are (generally) aware of these issues and openly discuss things like gay rights, trans issues etc.
Lets not forget these are still children and the way people have been talking about them is awful especially on social media. They are still learning, developing, growing, they are still forming boundaries, learning social cues etc. They will probably be unlikely to grow up and mention "I'm trans and LGBT+" as a first introduction, just as much as the girl who said she liked food and sleeping won't be saying that when she's 20 and meeting people for the first time.