Teenage thugs brandishing air guns fired shots in Lancashire secondary schools twice in the past two years, it has been claimed.
The attacks in three schools around Blackburn were revealed last week as the National Association of Schoolmasters Union of Women Teachers condemned escalating violence by pupils.
National executive member Dick Greenfield told the union's annual conference of three incidents in Lancashire schools within the past two years.
"The police were not involved, the incidents were swept under the carpet and the teachers were fearful of the consequences for their career if they spoke out," he said.
He claimed the attacks took place in two schools near Blackburn and at one in the nearby countryside. Guns were fired twice, but no one was injured.
Mr Greenfield claimed the teachers were pressed by senior managers in their schools not to report the attacks, but they had informed the union in an attempt to highlight the disruption in schools.
The NASUWT has pressed consistently for more effective sanctions against uncontrollable pupils and claims incidents are rising.
It has fought 46 cases so far this year of teachers seeking backing to refuse to teach disruptive children allowed back into school after appeals from their parents, compared to 40 in 1994.
Nigel de Gruchy, NASUWT general secretary, said: "The first priority is to protect a school, teachers and children."
He said violent children should be allowed to truant if that was the only alternative to them assaulting teachers or disrupting classes.
The NASUWT conference heard of a contract alleged to have been drawn up by education psychologists which proposed that a 10-year-old boy should be allowed back in school if he agreed to limit his attacks on classmates.
Jared Johnson, NASUWT secretary at Ashington, Northumberland, said the incident happened at a middle school in the county.
Under the proposed contract the boy was to be allowed back in class if he agreed not to hit pupils more than seven times a week.
Northumberland County Council said the contract was merely a draft for discussion. It had been made clear that violent behaviour was totally unacceptable, and exclusion was still a possibility.