Teachers are said to be puzzled by falling standards of handwriting, spelling and punctuation among S2 pupils.
The decline is likely to be confirmed by the latest Assessment of Achievement Programme, whose survey of English language is to be published later this year.
Elisabeth Sharp, English adviser in Grampian and secretary of the Association of Educational Advisers in Scotland, told The TES Scotland: "It is a matter of concern."
While many pupils were doing well and taking a pride in their work, some were experiencing problems, Mrs Sharp said. She added: "It is difficult for teachers to give them the degree of attention they need because of class sizes."
A shortage of learning support teachers was another factor.
The AAP sample of pupils in P4, P7 and S2 is expected to show progress in listening, talking and reading, despite continuing problems with writing, highlighted in the 1992 survey.
Researchers at Edinburgh University, in a section on technical skills, commented: "While some pupils were very competent, quite a large number at all stages still have some weakness in spelling, punctuation, linkage and paragraphing."