The correct response (if homeschooling is not an option) is a premature but detailed lesson on the real world:
Some adults lie about important things.
Some adults are mistaken about important things.
No, boys cannot become girls, either by choice or by accident. There is nothing to worry about.
No, girls cannot become boys, either by choice or by accident. There is nothing to worry about.
Boys and girls are not allowed to ask questions about this at school because sensible questions will get them in trouble with people who enjoy getting angry.
Can you tell whether you are a boy or a girl? How? Your classmate doesn’t understand this, and because their parents are also confused, you will get in trouble if you try to explain it.
I could go on, but the best response to wrong information is the truth, in all the detail that is necessary.