A model performance management policy, to be published soon by the Department for Education and Skills, provides for schools to either ban all staff other than the head from dropping in on classes or to limit the number of drop-ins.
The head will not be allowed to make judgments about teachers' performance and pay.
Instead, the head must schedule no more than three hours each year of formal, agreed classroom observations for performance management.
Though heads' unions agree that visits should not be oppressively frequent, they insist that school leaders must have the freedom to monitor the quality of teaching and learning in their schools.
Victoria Bishop, the head of Sir Christopher Hatton comprehensive in Wellingborough, said she and her leadership team "popped in" to classes as often as humanly possible and her new year's resolution was to do it even more.
"I walk around the school every day, looking through windows and doors,"
Mrs Bishop said. "I've walked around today with a senior science teacher and walked into classrooms completely unannounced and no one has expressed an objection to me."
Teachers' unions say heads are using formal observations and informal drop-ins to harass teachers.
The NUT is in a battle with a school in Newham, east London, about the head's "oppressive" use of classroom observations.
Steve Sinnott, the union's general secretary, said school leaders should not be able to enter any classroom more than three times a year, for formal observations and informal drop-ins combined.
A motion to the NASUWT national conference in April calls for a complete ban on unannounced drop-ins. Chris Keates, the general secretary, said observations had featured in half a dozen of its school industrial disputes in the past year, including the Ridings school in Halifax.
Only six months from the new regime taking effect, the partnership of government, employers and unions is still in heated discussions over how teachers' pay and performance will be measured. Nigel Middleton, director of the Head Support consultancy, accused the teachers' unions of watering down heads' authority, with little resistance from the Association of School and College Leaders.
Both heads' associations expressed optimism that any differences between heads and teachers could be resolved. John Dunford, the ASCL general secretary, said the new regulations would provide for clearer performance management.
Mick Brookes, the NAHT general secretary, said his union did not want teachers unduly harassed by heads who were anxious about Ofsted. "But we will not sanction any move to prevent headteachers, and other people responsible for teaching and learning in the school, from being in classrooms from time to time," he said.
BUT WHO KEEPS AN EYE ON THE BOSSES?
From September, new performance management regulations require heads to be formally observed in the classroom, "where appropriate".
Subject co-ordinators and department heads already observe their head's teaching to ensure that individual subjects are being taught well, but the head's pay packet does not depend on the verdict.
Most heads, especially in primary schools, spend between three and 12 hours a week in front of a whiteboard.
It is expected that most boards of governors, who review heads' pay and performance, will avoid a sticky situation by getting in a qualified teacher from outside to observe the head. Alternatively, they may decide - especially in bigger schools - that the head should be judged solely by their leadership of the school, not by the few hours they spend in the classroom.