Inspectors reported today that advanced skills teachers, who receive an extra pound;5,000 a year to spread good practice, had significantly improved the quality of teaching and learning in more than three-quarters of schools.

The number of ASTs has rocketed since the scheme was launched in 1998, rising from fewer than 1,000 in January 2002 to more than 3,400 this month.

The Office for Standards in Education surveyed more than half of the ASTs in post last year and found that they were generally very hard-working and highly appreciated by schools. But inspectors said that several issues had to be addressed if the initiative was to maintain its momentum.

Among the concerns were:

* Many were taking an “unacceptably long time” to visit other schools;

* The schools that most appreciated AST’s - those that were failing or had serious weaknesses - had the greatest problems recruiting them;

* Primary heads had difficulties finding supply cover when they visited other schools;

* Some schools in challenging circumstances had unreasonable expectations of what they could achieve in a short time.

One AST who has no difficulty visiting a variety of schools is dance teacher Sarah Waller, who works with different clusters of primary and secondary schools each month. Unusually, she is not employed by a school but by Cornwall Outdoors, a project run by the county’s education authority.

“You have to be driven and passionate to be an AST,” Mrs Waller said, “But I don’t think it’s right to call us ‘superteachers’. It’s flattering, but it’s embarrassing.”