The comments were made ahead of the union's annual conference next week where the "prevalence of sexist bullying and harassment" of students in schools will be discussed.
Dr Mary Bousted, general secretary of the ATL teaching union, said: "I think sexist bullying is the thing that doesn't get talked about in school.
"I think for girls there is a very fine line between if you're swotty and clever and you answer too many questions [then] you're not attractive.
"If you were a girl, particularly an adolescent girl, there are so many names you can be called in school. There are very few for boys. It's very hard for a girl to be brainy and feminine."
She told journalists in a briefing before the conference that there was a "very big pressure" in all schools to "keep quiet and to listen to the boys talking".
But Dr Bousted (pictured) stressed that there was also a "hierarchy" in single-sex schools. "You still get that sorting into the brainees, the swots, and the ones who like boys," she added.
A motion, to be heard at the conference in Liverpool on Tuesday, calls for members to be given greater assistance on how to tackle the problem in schools.
Dr Bousted added: "Schools of course have to promote equality and respect between the sexes and promote the behaviours that they want to see in their school. But schools can’t tackle this on their own. This is an issue for society."
But the union leader stressed that all pupils should be taught how to speak up in school and she believes debating societies – with equal gender representation – could help.
She said: "My worry is that we have a curriculum with timed written exams – which actually has taken speaking and listening out of English GCSE and that’s a core skill…The thing that people judge you on straight away is being downgraded in schools."
Dr Bousted also expressed concerns over the problem of "sexting" in schools after recent research into sexual bullying in schools identified children as young as seven had been caught sexting by their teacher.
The survey by the NASUWT last week showed more than half of all school staff in the UK are aware of pupils using social media to share sexual messages, pictures and videos.
Dr Bousted added: “The victims of [sexting] are overwhelmingly girls...adolescents today have more access to highly sexualised videos, films, content on social media than ever before and it would be surprising if that didn’t play out in one way or another in schools.
"It is very difficult for teachers to police that. You can’t just confiscate everybody’s mobile phone.”
Want to keep up with the latest education news and opinion? Follow TES on Twitter and like TES on Facebook