A heads' leader is warning members against taking part in joint lesson observations with Ofsted inspectors.
Mick Brookes, National Association of Head Teachers general secretary, is concerned that by taking part in the exercise, heads could needlessly open themselves up to criticism from the watchdog.
New Ofsted guidance tells inspectors to record the accuracy of heads' views in such observations. That evidence can then be used to help form judgments on the quality of leadership in a school.
"Any judgment of classroom teaching is subjective," Mr Brookes said. "If the stakes are raised on this, then it would cease to have any merit and should be discontinued.
"My advice to members is, if this evidence is likely to be used against them, then maybe it is something you don't want to do."
Joint lesson observations are one aspect of a drive to encourage greater involvement from senior school staff in the inspection process from September. But staff participation is voluntary.
John Dunford, Association of School and College Leaders general secretary, said he saw the process as a chance for heads to show the quality of their self-evaluation.
He argued that they had nothing to lose because, if their judgments were wrong this would emerge anyway through their self-evaluation forms.
"The point of joint lesson observation is that we want quality assurance in schools to centre around self-evaluation," he said.
But Mr Brookes said it would "knock the stuffing" out of the bid to build better relationships between heads and inspectors.
The new framework being introduced next term will also enable heads to attend inspectors' meetings so that they can hear how judgments on their school are reached.
An Ofsted spokesman said: "Joint lesson observations have been widely welcomed by schools in pilot inspections. They are designed to be of benefit to the school in supporting school improvement.
"Whether headteachers choose to participate in joint observations or not, inspectors need to form an opinion about the accuracy of a school's self-evaluation."
Last month, the watchdog defended the guidance, saying inspectors should avoid observing "weak" teachers already identified by heads.