Bad behaviour in schools is being fuelled by "overindulgent" parents who don't know how to say no to their children, according to new research. Teachers are dealing with a "small but significant" number of pupils who throw tantrums in class if they don't get their own way, turn up exhausted because they stay up late and have increasingly "belligerent" parents who take their child's side.

"These parents, themselves under social pressure and often unable to deal with their own children's behaviour, could be highly confrontational, sometimes resorting to violence in protecting their children's interests," the Cambridge University study, commissioned by the National Union of Teachers, said. "Teachers described 'highly permissive' parents who admitted to indulging their children, often for the sake of peace or simply because they had run out of alternative incentives or sanctions."

Steve Sinnott, NUT general secretary, said parents were under increasing pressure because of the commercialisation of childhood. "Some parents are struggling to cope with their children. Parents seem to be trying to cope by indulging their children, and perhaps more accurately overindulging their youngsters," he said.

The Cambridge University study revisited primary schools which were involved in a study five years ago. In 2002, schools reported that their greatest problem was trying to control classes bored by a narrow and unchallenging curriculum.

Last year the same schools told the researchers they were getting better at navigating the constricts of the curriculum to engage pupils, but were facing new behaviour problems in the face of a "rapidly changing social scene".

"Faced with deteriorating standards of behaviour and a more belligerent stance from some parents, primary schools were becoming more like secondary schools, instituting formal recording systems whereby incidents were logged and behaviour monitored," it said.

Interviews with 200 teachers, and hundreds of pupils and parents revealed families struggling to control their children. One mother said she considered the fact that her son went to bed at 1am a success as previously he had refused to sleep before 3am; and a six-year-old told his teacher how to go about "killing pimps and prostitutes" after mastering the Grand Theft Auto computer game.