Some teachers are being "observed to death" by senior staff, heads' associations admit.
The issue of lesson observation is set to be one of the most controversial topics at next week's classroom union conferences.
Members of teachers' union the NASUWT and the National Union of Teachers will complain that senior staff are using the growing use of self-evaluation as an excuse to act like inspectors. Their concerns have been acknowledged by headteacher unions, which admit the practice has got out of hand in some schools.
Stephen Szemerenyi, an Association of School and College Leaders (ASCL) field officer and pay and conditions consultant, said greater care was needed about the way in which heads observed their colleagues.
"Some heads don't know what it's like. Some teachers feel they are observed to death," he said.
The ASCL, formerly the Secondary Heads Association, has agreed guidelines with the NASUWT, so its members should only observe teachers for a maximum of three hours per year.
A more formal agreement on time limits for classroom observation is due to be discussed shortly by unions in the Reward and Incentives Group. John Dunford, ASCL general secretary, said that heads were under greater pressure to observe lessons because of the new systems for inspections and school self-evaluation, which focus more on teaching quality.
Classroom unions said they were encouraged that heads recognised the dangers of excessive observation.
Mary Bousted, ATL general secretary, said: "Self-evaluation has become self-inspection. Lesson observation is often being done in a punitive rather than empowering way."
A survey by the NASUWT earlier this year indicated that a fifth of teachers were observed more than six times last year.