While actual or threatened violence against headteachers remain rare - only eight cases have been reported to SHA since January - the union says growing numbers of parents are challenging schools' decisions. More than 240 calls concerning parents were made to the members' hotline last year, up 10 per cent.
Rows are most likely when pupils are disciplined or excluded, but can arise if teachers tell parents their children are underachieving, or parents feel heads have failed to calm their fears about issues such as bullying.
And some public-school parents who think their fees guarantee good GCSEs are turning to their lawyers when their children fail to score straight As.
The Centre for Policy Studies warned this week that Britain's growing "compensation culture" was costing the country almost pound;7 billion a year in lawyers' fees and damages. Hospitals and local authorities are typical targets of law suits.
Threats of violence and lawyers' letters both made balanced decisions in the interests of the pupil difficult, SHA's deputy general secretary, Russell Clarke, said. "Bringing solicitors to meetings ups the ante and makes them confrontational," he said.