Popular cartoons, such as Ben 10 and Power Rangers, are to blame for a rise in playground violence among young boys, say teachers.
Deep concerns about the influence of television on children's behaviour have been revealed in a survey conducted by the Association of Teachers and Lecturers.
Half of the 800 teachers and support staff questioned, including many Welsh members, believed TV programmes were causing aggressive behaviour.
Children were said to be mimicking characters, such as cartoon hero Ben 10, by kicking and fighting in the playground. This often started out as a game, but primary teachers reported the "play-fighting" was increasingly getting out of hand.
Ben 10, a cartoon in which 10-year-old Ben Tennyson transforms into 10 alien characters to fight baddies, is now one of the most popular cartoons among primary school boys.
The accompanying computer games, which involve players delivering punches and kicks at the flick of a key or mouse, are also commercially successful.
ATL members surveyed in England and Wales said television was overwhelmingly the biggest influence on how children act, with computer games a close second.
Children were also copying the language they heard on such shows, including crude catchphrases such as "kick butt" - much-used in Ben 10 - and swearing, teachers said.
Elsewhere in the survey, a teacher at an independent secondary in Wales said older pupils had quoted sketches from comedy programme Little Britain "loud enough for me to hear about breast-feeding".
Three-quarters of those polled felt TV programmes should be classified by age, like films.
Dr Philip Dixon, director of ATL Cymru, said teacher members found rudeness, swearing and aggressive behaviour were now the norm not the exception. "It's time for those who make programmes to take into consideration factors other than just their ratings: we should be asking if programmes are an influence for good or for bad."
Next week, teachers at the union's annual conference in Liverpool will decide whether to lobby broadcasters about bad behaviour shown on TV before the 9pm watershed.
Dr Heledd Hayes, education officer for the NUT Cymru teachers' union, said schools were concerned about violence, but that the underlying cause might not be as simple as TV or computer games.
"I'm often very impressed by how young people do realise what is appropriate," she said. "But I certainly think young children don't understand why in programmes such as Tom and Jerry the cat gets hit and then gets up again."