New powers to detain pupils without parental notice, given to schools by the coalition Government, are unlikely to be regularly used, according to heads.
Teachers have said they are worried about being blamed if something happened to the child while they were kept away from home without parental consent.
The changes are part of "relentless focus on school discipline" by the new Government, which wants to "deregulate" punishment.
Education Secretary Michael Gove's proposals - to change the law this autumn so teachers will no longer have to give 24 hours' notice before running after-school detentions - are part of a wider overhaul of behaviour regulations.
On Wednesday ministers announced new powers of search and restraint, including the ability to confiscate "legal high" drugs, pornography, cigarettes and fireworks.
But Mike Griffiths, head of Northampton School for Boys, said he doubted whether most teachers would actually use the new detention powers.
"I'm sure most teachers would think it very unwise not to inform parents about a detention," he said. "They would be the first to be blamed if something happened to one child going home alone afterwards without their families knowing where they were."
Lesley Gannon, assistant secretary of heads' union the NAHT, agreed. "All heads will be very cautious of anything which could jeopardise their relationships with families and I think the majority will feel they have sufficient powers," she said.
Nick Gibb, schools minister, said the changes were designed to "deregulate" punishing children.
"It is not a prescriptive matter requiring schools not to give 24 hours' notice for detentions: it merely enables them to do that if they wish," he told MPs.
"Trusting headteachers and teachers means that they will make these arrangements themselves if schools feel they are necessary. We are trying to take out of the statute book impediments to maintaining good order and good behaviour in our schools."
The new Government will also introduce measures, first outlined in the Conservative party manifesto, to give teachers facing "potentially malicious" allegations anonymity. Complaints will be "investigated within a tight time period or dropped".
Mr Gove will make a further announcement about behaviour later this month, which will include changes to independent appeals panels and pupil referral units.
The Education and Children's Bill will give heads more powers to expel pupils, but it is as yet unclear exactly what they will be or whether the appeals panels will be scrapped.
Heads' unions are lobbying the Education Secretary to keep them.