With screams of "kick him" and "stamp on him", some fathers and mothers of pupils at independent schools are transformed by the thrill of the game. Now guidelines have been set out to curb wayward parents.
"They have been heard shouting at the referee: 'Get yourself a pair of glasses' and calling on their children to attack him," says Vivian Anthony, secretary of the Headmasters' and Headmistresses' Conference. "When their excitement goes over the top, they have been advocating violence rather than shouting encouragement."
It is not only parents whose wild behaviour at school sporting events is causing concern: pupils and their coaches have also come under fire.
Professional coaches are believed to be perverting the sport by importing "sledging" - an Australian term for verbal intimidation. In school cricket matches, pupils have been seen hemming in the batsman, as in professional cricket.
"It is possible that some of these practices have filtered in through schools where there are professional coaches, but heads must remember that there are standards which must be observed," said Mr Anthony.
Heads are warned there should be no foul or abusive language on the pitch and that pupils who persist in shouting abuse should be banned from taking part in sport, according to the guidelines written by Chris Hirst, head of the conference's sports sub-committee.
Parents are not let off lightly by Mr Hirst, headteacher of Sedbergh, the school attended by Will Carling, former captain of the England rugby union team.
He reminds heads that schools are responsible for the behaviour of parents on the pitch. Although quite what should be done with adults screaming obscenities, he leaves to the imagination.