School support staff complained this week that they were being routinely bullied and intimidated by "demon headmasters".
The national conference for Unison, which represents around half of Britain's school support staff, heard that some were forced to teach classes or work extra hours without pay by heads who told them that children's education would otherwise suffer.
Others had phoned union representatives for help from inside cupboards because they were scared to be caught complaining.
Paul Holmes, a Unison representative from Kirklees, told the conference in Bournemouth that most schools were "run on fear" by the headteacher.
"They could be renamed 'The Lion, the Witch and the Wardrobe' because the calls I get are from cupboards and wardrobes," he said.
Sheila Hemingway, from Leeds, said staff were intimidated by "demon headmasters".
"Support staff in schools are terrified," she said. "They're working some of the longest unpaid hours in the country, and they are scared to say anything because of the repercussions."
Jonathan Sedgebeer, from Telford, said heads bullied and harassed support staff and used them as dispensable commodities when money got tight.
Ministers agreed this year to a negotiating body that would cover most of England's estimated 500,000 classroom and learning assistants, caretakers, cleaners and dinner ladies.
But members of Unison, which represents 200,000 school support staff, argue that the plans do not go far enough and that the body should also include teachers.
Christina McAnea, the union's head of education, said a national pay deal would go some way towards stopping schools and local authorities taking advantage of such workers, who are among the worst-paid in the public sector.
Some support staff are paid annual salaries as low as pound;15,000, and that is reduced further because many schools and local authorities do not pay them during school holidays. Camden council in London gave all its staff a paid day off to thank them for their hard work - except school workers because it was "too complicated".
And with a tight three-year school funding round beginning this year, support staff are the most vulnerable to redundancies when heads are forced to lay people off.
Mrs McAnea said: "There's a lot of moral blackmail going on in schools. Support staff are made to feel that if they don't work the extra hours on residential trips without pay, don't cover classes for absent teachers at short notice, then the children will suffer."
Unison's leadership has promised to consult with members and other unions about whether the pay negotiating body should be expanded to include teachers as well - which would establish a formidable school negotiating bloc of one million workers.
Strain on leaders with a job to do
Heads' associations suggested that the support staff in Unison were being over-sensitive.
John Dunford, general secretary of the Association of School and College Leaders, said heads' actions in ensuring people did their jobs properly were sometimes interpreted as bullying. "Heads are under enormous pressure to run successful schools, and sometimes this will mean people have to change their working practices, even though they may be unwilling to do so," he said.
Mick Brookes, general secretary of the National Association of Head Teachers, said it was a mistake for Unison to portray all heads as bullying demons. "I think that is a complete fiction," he said. "It would be reprehensible if any headteachers were acting in this way, but it doesn't happen at the vast majority of schools.
"Nobody gets paid to go on school trips - not teachers, not governors, not parents. If support staff were not interested in going on school trips, then frankly I, as a head, wouldn't want that sort of person on a school trip I was running." - Jonathan Milne.