Jimoto no Ijimekko-tachi ni Shikaeshi Shiyou to Shitara, Betsu no Tatakai ga Hajimatta. - Vol. 1 Ch. 8 - The Battle Against Unwanted Hair Begins.

[SonicDisgaea98]

If they were childhood friend, how is this guy old enough to be a teacher?

 

[isoycrazy]

If they were childhood friend, how is this guy old enough to be a teacher?

He could be 2-3 years older than the 4 idiots, and so just became a teacher.

 

[Cyromantik]

...I mean, either they're transexual and the school is VERY progressive, or the translators made a mistake translating the way the teacher refers to it as "(their) daughter", which is very much NOT ambiguous in Japanese, usually...

So no, they're most likely a girl.

Public school in Japan is pretty progressive though, well somewhat depending on prefecture. They're LGBTQIA+ aware and some have relaxed the dress code so that either presenting boys or girls can wear whichever uniform they wish. However I've only ever seen the girls wear boy's uniforms though. Some of the boys wear barrettes and use feminine pencil cases so it seems the AFAB folk have it a little easier still, but they're on the right track