Since April inspectors have been required to grade teachers from one (best) to seven (worst) and to report those given the two bottom grades to the head.
Peter Miller, new president of the Secondary Heads Association, said inspectors appeared reluctant to give teachers grade six and seven.
"We are seeing a lot of teachers graded five," he said. "Quite possibly one of the reasons we are seeing a lot of grade fives is that inspectors are choosing to avoid the hassle, which is quite understandable."
If inspectors wished to give teachers the worst grades, they had to watch extra lessons and tension increased during the inspection, he added.
Mr Miller said heads knew who the bad teachers were and were undermined if inspectors failed to back them up in their efforts to deal with them.
"If a head is trying to help a colleague who is clearly struggling, or even taking disciplinary measures, and that teacher is not given a bad grade, the head is undermined."
Inspectors also seemed unwilling to name the best teachers, perhaps because they feared it would increase jealousy in the staffroom, he said.
Mr Miller, deputy head of Wrenn School, Wellingborough, Northamptonshire, said the new rules were misguided and would do more harm than good. "This is a distortion of the inspection process."
Chris Woodhead, the Chief Inspector of Schools, has said that 15,000 teachers are not up to the job.
A spokesman for the Office for Standards in Education (Ofsted) which Mr Woodhead heads said it was too early to decide yet whether the new system was working. "It goes without saying that we expect registered inspectors to use the full range of the marking scheme and we take a dim view of teams who are failing to do so.
"Good teachers deserve to be identified and headteachers are entitled to the valuable management information which we said the system would yield. If it is not yielding that information, it is not doing the job it was intended to do."
Gillian Shephard, Secretary of State for Education, announced yesterday she was accepting recommendations from Ofsted that most schools should be inspected once every six years instead of every four years as at present. Weaker schools will be visited more often.
t Four teachers' unions yesterday called for a pay rise above inflation and average earnings and for a legal limit on class sizes. The four unions say in their submission to the School Teachers' Review Body that they want a "substantial" increase. Inflation is running at 3 per cent and average earnings are expected to be up by 4.6 per cent by March next year.
The unions want a class size limit of 30 with lower limits for mixed age, reception and practical classes. More than a million primary school children are in classes of more than 30.
The Association of Teachers and Lecturers, National Association of Schoolmasters Union of Women Teachers, National Union of Teachers and Professional Association of Teachers, say that, outside teaching starting salaries for graduates have risen to pounds 15,000 while for teachers the figures are pounds 13,866 for good honours graduates and pounds 12,342 for others.Reuse content