Scottish headteachers have launched an investigation into the Higher English exam amid concerns that too many pupils are not getting the results predicted for them.

Most secondary schools in Scotland are being surveyed by a headteachers' union to find out the proportion of pupils who missed out on their expected grades in English - and by how much. Maths is also being looked at to provide a benchmark.

The Headteachers' Association of Scotland (HAS), which represents secondary heads, is conducting the exercise to find out whether anecdotal concerns about a "surprising" divergence from predicted grades are correct.

Although overall pass rates in English have remained stable over the past few years, teachers feel that there is too great a spread between what pupils are predicted to get and what they achieve - in some cases dropping more than two grades.

There are also concerns that some candidates who are predicted to get lower grades do much better than expected.

Concerns first came to light two years ago after several candidates who got straight As in all other Highers got only a D or C for English, despite being predicted an A. When the cases were discussed informally by headteachers, similar stories emerged about other candidates.

The HAS raised the issue with the Scottish Qualifications Authority (SQA), Scotland's exam body, but were told there wasn't enough evidence to support the claims.

The HAS decided to conduct its own survey to try to establish whether the trend is genuine.

The current question mark over the Higher English exam is the latest in a string of concerns about the qualification, which has already been through several revisions since the introduction on Higher Still in the mid-1990s.

In 2001, a review of the exam described elements of it as "unworkable" and "undeliverable" and called for a drastic reduction in the burden of internal assessment.

Although changes were made, in 2003 ministers came under fresh pressure to revise Higher English after the pass rate dropped for the second consecutive year. Teachers and academics called for creative writing to be incorporated into the exam, saying the latest version didn't test the most talented pupils.

However, the SQA argued part of the problem was that teachers were entering pupils for Higher when they should have been sitting lower levels of the course such as Intermediate 2.

Last night, an HAS spokesman said: "There have been concerns in the past about the reliability of Higher English results, which appear to display a surprising level of variation from departmental estimates. We wish to collect as much data as possible to determine if our concerns are justified and, if so, pass this hard evidence to SQA."

An SQA spokesman said: "We welcome all feedback, particularly from education professionals such as the members of HAS.

"We have a duty to ensure that Scotland's qualifications system remains fit-for-purpose, and we will always look carefully at information like this. We would then strive to take whatever action is deemed appropriate."