Dr Mary Bousted, general secretary of UK teaching union the Association of Teachers and Lecturers, said the prevalance of sexist bullying and harrassment of students is "as great now as it ever has been".
As a result, it is even harder for girls to appear bright in class without being targeted, Dr Bousted added.
"I think sexist bullying is the thing that doesn't get talked about in school," she said ahead of her union's forthcoming annual conference.
"I think for girls there is a very fine line between if you're 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."
Pressure on girls
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 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."