The 'outstanding' grade used by Ofsted creates a "false impression" about the quality of schools and carries too much influence in the education system, according to a leading head.
Gary Phillips, of Lilian Baylis Technology School in south London, which was rated outstanding across the board in its most recent inspection two years ago, said that the judgment carries too much weight and suggested it was time for an overhaul of inspection grades.
Mr Phillips also argued that it was time to consider removing the "inadequate" rating due to the consequences of schools receiving the judgment. Instead, schools could be judged "not good enough", he suggested at a conference on inspection today.
Under plans announced by Ofsted chief Sir Michael Wilshaw, from this autumn schools previously rated good by the watchdog will get short inspections every three years to check that they are still performing well. The move came amid concerns that leaving up to seven years between inspections of good schools is too long to spot declines in standards.
But the overhaul does not apply to schools handed an outstanding rating, as they are exempt from regular inspections unless there are particular concerns.
Speaking at a Westminster Education Forum on school inspection, Mr Phillips said he believed that planned changes to inspections, which also include an end to contracting out inspections, are a move in the right direction.
But he added that there are two areas where Ofsted's proposals do not go far enough. "Firstly, grading," he said. "Presently, there are four grades, from inadequate to outstanding. However, they're not very useful in many ways."
Once a school is classed as outstanding, there is a belief among parents that it is outstanding in every way, on every day, Mr Phillips said.
"Put simply, the outstanding judgment creates a false impression," he said. "Perhaps more worryingly, it carries far too much weight in the education system. It's used for the designation of national leaders of education, teaching schools and it has a huge influence on academy sponsors being identified.
"System leadership should be more robust in identifying those who are going to take bigger roles than using often outdated inspection reports.
"This over-reliance on the Ofsted grade of outstanding is a concern when you consider there are 1,200 outstanding primaries and 150 outstanding secondaries that were last seen by Ofsted before September 2009.
"This means that none of these schools have been inspected in detail against four new Ofsted frameworks since September 2009. And yet they all carry the designation of outstanding against a pre-2009 framework."
Mr Phillips added: "The second judgment that needs some consideration to be removed is the inadequate judgment. A judgment of inadequate is dramatic; it's far-reaching for schools and, in most cases, the school then goes into freefall after receiving the judgment and can take up to two to three years to recover. In some cases, longer."
During this period, there are often problems with staffing, and pupils' self-esteem and their belief in the power of education can fall, Mr Phillips suggested.
"As a result, there's a further erosion in outcomes for those children in the school and it usually has the greatest impact on the poorest communities," he said. "What is really needed is one 'not good enough' judgment rather than two that we have at present. This would reduce some of the unintended consequences we now see in those schools that are found inadequate."