Girls are keeping quiet during classroom debate because they would prefer to be considered attractive rather than intelligent, an education leader has warned.
Dr Mary Bousted, general secretary of the Association of Teacher and Lecturers (ATL), said sexist bullying remains prevalent in classrooms across the country, where girls are subjected to name-calling and insults for being smart.
Speaking ahead of the ATL annual conference in Liverpool next week, at which the subject of sexist bullying and harassment of students is due to be discussed, Dr Bousted said historical, societal attitudes to women were becoming internalised by female students.
She said: "I think sexist bullying is a thing that just doesn't get talked about.
"For girls, 'if you are swotty and clever and answer too many questions, you are not attractive'. There's a very fine line.
"There is a conspiracy of near-silence amongst girls."
The former English teacher said she once taped the lessons at her London comprehensive school believing the split of boys and girls speaking during class discussion was a "fairly even split". However, listening back to the recording, she said it became clear "the boys were talking and the girls were listening".
She said: "If you are an adolescent girl there are so many names you can be called in school. It is very hard for a girl to be brainy and feminine.
"It can make achievement at school very hard, it can become hard for them to believe they can achieve."
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.
ChildLine said it dealt with more than 1,200 concerns last year from young people worried about indecent images they had shared, or who felt pressured into sexting.
Dr Bousted said: "The victims of sexting are overwhelmingly girls.
"I think schools are doing what they can. They are often the safest places for young girls to be, but they are also chock-full of people who bring the (offensive and sexist) names which are outside school.
"Adolescents today have more access to highly sexualised films and content, on social media, than ever before. It would be surprising if that didn't play out in schools.
"But I am very confident that schools are dealing with this much better than they did a few years ago."
She added: "This is not something particular to schools. Schools have to promote equality, but schools can't tackle this on their own, it is one for society.
"I think all pupils should be taught how to speak up."
The ATL conference runs from April 4-6.