A school praised by the Government for being one of the most improved in the country is failing its pupils according to Ofsted inspectors. Cockburn College of Arts, in Leeds, is involved in a bitter dispute with the school standards watchdog after inspectors judged it to be sub-standard and issued it a “notice to improve”.

But while the school waits for its appeal verdict, it has been invited to two receptions to celebrate its achievements - one attended by Alan Johnson, the Education Secretary, and the other featuring Christine Gilbert, the head of Ofsted, as its guest of honour.

Inspectors, who visited in November, were heavily critical of pupil attainment, describing it as “exceptionally low” and among the worst in the country. But the Specialist Schools and Academies Trust has praised Cockburn for boosting its results over the past three years and admitted it to its “most improved club”.

Colin Richardson, the headteacher, was invited to a trust drinks reception with Mr Johnson earlier this week and to a formal celebration dinner with Ms Gilbert next month.

In March last year, Jacqui Smith, then schools minister, wrote to the school to applaud its achievement as the ninth most improved in the country for key stage three.

“It’s a travesty of justice,” said Mr Richardson, who has been headteacher for 14 years. “The whole situation is just crackers. How can we be both one of the most improved schools in the country and then be given a notice to improve? We are already on an upward trend. Results are improving year after year.”

Mr Richardson is critical of the inspectors, whom he says had made up their mind beforehand.

“On the first morning, the lead inspector said that... we would either be put in special measures or given a notice to improve,” said Mr Richardson.

“That was before she knew anything about the school apart from some data.”

He said the inspectors had reached their decision based on the school’s contextual value added (CVA) scores for 2001 to 2005, which measure pupils’

progress while taking into account their educational and social backgrounds. Although inspectors are supposed to refer to the scores, Ofsted’s guidelines require that they make up only part of the overall judgment.

The school considered taking the case to judicial review, but decided against because it would cost pound;25,000. A first appeal backed the inspectors’ findings but the verdict of a second is awaited.

Mr Richardson believes the appeal process was stacked against the school because staff were not allowed to meet those investigating the complaint.

An Ofsted spokeswoman said: “It would not be right for us to comment publicly on a complaint we’re still investigating.”