New light-touch inspections have led inspectors to become over-reliant on faulty performance data, with insufficient time in schools to spot the mistakes, the Association of School and College Leaders said.
Ofsted has been forced to apologise to heads over serious errors in the performance and assessment report, or Panda, sent to secondaries across England. Heads are also angry about what they see as an inconsistency between teams that inspect schools.
In a separate move, Ofsted has warned inspectors against making inappropriate requests for information after a secondary head was asked to grade her 94 staff on a scale of one to four at two days' notice. Schools judged inadequate face re-inspection within a year. If they fail to show improvement, they are placed in special measures.
The Government promised a crackdown on failing schools in last year's white paper, with those who fail to improve being closed within 12 months.
A new inspection framework was introduced in September to cut costs and reduce the burden on schools by placing greater emphasis on schools'
The latest results are expected shortly. Ofsted admitted mistakes in its contextual value-added Pandas after A* grades were omitted from calculations to determine the value-added by the school. Also, schools were not given credit for the GNVQ scores of Edexcel exams.
Several schools believed they had wrongly been given an official "notice to improve" - formerly known as serious weaknesses - as a result of the mistakes.
John Dunford, general secretary of ASCL, said: "The lesson from autumn 2005 is the same as from autumn 2003. Make sure you don't get inspected in the first term of a new framework."
Peter Shaw, head of Stainburn school, in Workington, Cumbria, who complained to Ofsted about the data, said: "If I had made this sort of error as the head of a school, inspectors would fail me."
Pandas are the key tool for inspectors in forming pre-inspection opinions about the quality of a school's performance. Heads are also expected to use them to set their priorities for improvement.
An Ofsted spokeswoman said inspectors had been made aware of the mistakes, which were the result of a programming error and the late arrival of GNVQ results for about 350 schools.
She said the inspectorate would correct these mistakes in two further Panda reports planned for this term.
A revised version of the Panda, which will include the A* grades and also indicate whether a school's data has been affected by the GNVQ omissions, should be available from January 20 or shortly after. Full details are available at the secure website: www.ofstedpandas.gide.net