Mr Walker has announced he will retire at the end of the spring term

Peter Walker, 59, joined the Abbey School in Faversham, Kent, in 1987.

He began the controversial testing of pupils for drugs at the start of this year and has said it helped the school achieve its best-ever GCSE results.

He said he would retire in April 2006 to improve his quality of life, adding: "There is no motive other than that."

I don't like the selection system and I have never made any secret of that Peter Walker

The Abbey School is a non-selective school with 960 students aged 11-19.

This summer, 40% of pupils achieved five good GCSE passes, compared with 26% last year and 32% the year before.

The improvements had made his decision to go easier to make, he said.

"It is always best to go out at a good time rather than go out under a cloud and it was a contributory factor," Mr Walker said.

"But I planned my retirement when I entered the profession all those years ago. I planned absolutely everything and gave myself targets from the age of 18 when I started teaching."

'Improved tremendously'

He said he believed the education system now was better than when he started out.

"Things have improved tremendously in Kent and the county has an education success to be proud of in many ways.

"The bit that I don't like - and I have never made any secret of that - is the selection system but I have always worked within it," he said.

"I made a decision to come here knowing there was selection and I could have chosen to get a job in a grammar school but I preferred to work with children that I felt I had more affinity with."

During his career Mr Walker has been a spokesman for the National Association of Head Teachers and served on government advisory councils.

Originally from Yorkshire, he said he now wants to take on new challenges including travelling and writing.