The class of 2002 at popular Thornton Grammar School under-achieved in GCSEs due to the schools reorganisation, says watchdog Ofsted.

The school won praise in its new inspection report, which returned an overall verdict of "good".

But exam results dipped markedly in 2002 interrupting a period of year-on-year improvements.

Ofsted blames the disappointing show of 37 per cent of pupils gaining five A*Cs in 2002, on disruption caused by the reorganisation, when the school grew to 1,500 pupils.

"There is no doubt that the pupils taking their GCSEs underachieved," the report says. "The school was shocked by the results.... 2003 saw the school come bouncing back."

This summer, 48 per cent of students gained five A*-C grades, an all-time high for Thornton but still below the national average.

The report explains: "The dip can be blamed entirely on the reorganisation of education in the local education authority (LEA) and its subsequent problems for the school.

"The school virtually doubled in size, without the necessary accommodation as the building of new classrooms had not been completed.

"As a result, a significant amount of time from lessons was lost, as teachers and pupils had to make their way around a small village of mobile classrooms.

"A significant number of new staff.... came from middle schools.. and received no special training before taking up new posts."

Overall inspectors were impressed with teaching quality at the school, commenting on the fact that the "strict but friendly" staff make Thornton popular with parents.

They noted examples of outstanding practice, describing an excellent chemistry lesson that stretched even the most able pupils, and the way the school has used its designation as a specialist sports college to drive up standards. The school's efforts to combat the under-achievement of boys from a Pakistani background is another success.

Head John Weir - who has seen GCSE results double since his appointment - said: "We do think it's a pretty fair report which points us in the right direction for the future."