Abusive and violent parents are a growing occupational hazard for teachers, and escalating verbal and physical attacks are being blamed for high staff turnover.
In recent incidents parents have threatened to to "kill" and "cut up" teachers in Philadelphia. Staff have also suffered bloody noses and black eyes. said the city's deputy schools communications chief, Fernando Gallard.
In the six months to the end of February this year, 57 parent assaults on city teachers were recorded. "We're seeing more and more hitting of staff," said Mr Gallard.
Many of the parents had been summoned to school over their child's bad behaviour, Mr Gallard said. As far as they were concerned,"it's not their child's fault, it's the teacher's, and they're ready to rumble."
Nationally, staff say handling parents is a bigger headache than funding, pupil behaviour and the stress caused by tests, according to the MetLife Survey of the American Teacher. The strain of dealing with parents is also a major reason behind the fact that half of new teachers quit within five-years, says the Consortium for Policy Research in Education.
Last month, Philadelphia mounted a parental outreach drive to try to stem the rising tide of violence against teachers.
The "parent leadership academy initiative" will seek to galvanise positive parental engagement with schools, Mr Gallard said.
Inner-city parents may lash out because they mistrust schools, having dropped out themselves, said Annette Lareau, professor of sociology at Temple University.
But while assaults on teachers in poor urban communities are linked to unengaged parents, and domestic and community disputes spilling into schools, suburban schools also report a growing number of run-ins with affluent, pushy parents, who meddle in their work. Accountability reforms - holding schools up to public scrutiny - and efforts to promote parental involvement in schools are emboldening some parents, say experts.
Facing increased testing demands and funding constraints, schools are grateful to parents who volunteer to help by, for example, taking reading or coaching, said Linda Hodge, president of America's National Parent-Teacher Association.
But some teachers complain that parents are now crossing the line, questioning staff judgement and meddling in minutiae.
Middle-class parents rarely resort to fisticuffs but they will harangue staff about grades, said Professor Lareau.
"Many teachers feel under siege. They want parents to defer to their professional judgement."