British teachers will today call for an investigation into rising levels of poor behaviour among girls in the classroom.
Girls, they argue, are more likely to resort to cyber-bullying - bullying over the internet and mobile phones which is worrying school leaders.
A survey by the Association of Teachers and Lecturers reveals that half the teachers believed girls' behaviour had become worse during the past two years. One in five thought it was now more challenging than boys'.
Teachers interviewed for the survey accused girls of being "sneakier" than boys in the way they misbehaved.
They were also more adept at using modern social media such as Facebook to bully their fellow pupils.
The survey of 859 teachers in primary and secondary schools and further education colleges said girls' misbehaviour centred on isolating fellow pupils from a friendship group, spreading rumours and making snide comments.
"Girls spread rumours and fallouts last a long time," a 34-year-old teacher from Reading told researchers. "Boys tend to sort it out fairly quickly."
Another teacher drew attention to a rise in the use of social media to bully pupils. "There is a lot of cyber-bullying particularly via MSN and Facebook - this is mainly girls," said the teacher, who is a member of the senior management team of an English secondary school.
The finding follows a one-day strike by teachers at a high school in Lancashire this month over pupil behaviour.
Teachers complained pupils were making videos and taking photographs in class with their smartphones, then posting them on websites such as YouTube.
The teachers were particularly unhappy that senior management did not back them up when they confiscated pupils' phones.
Today's motion at the ATL conference calls for more support for teachers in instilling discipline.
One teacher told researchers that girls were also becoming more violent.
"Girls are definitely getting more violent with gangs of girls in school who are getting worse than gangs of boys," she said.
However, a primary school teacher from Bedfordshire added: "Boys are generally more physical and their behaviour is more noticeable.
"Girls are often sneakier about misbehaving, they often say nasty things which end up disrupting the lesson just as much as the boys as other children get upset and can't focus on their work."
The teacher added. "[The girls] are usually the ones who refuse to comply with instructions."
The feeling was, though - despite rising levels of aggressive behaviour among girls - that boys were still more likely to show physical aggression.
"Staff get ground down daily by the chatting and messing around, which disrupts lessons for other pupils and takes the pleasure out of teaching," said Mary Bousted, general secretary of ATL. "Even more worrying is the physical aggression - most often among boys but also among some girls - which puts other pupils and staff at risk."
The union's annual conference in Liverpool will today hear calls to investigate the number of girls being excluded from school. Delegates will be told it is an increasing problem.