Fairfax County School Supt. S. John Davis ordered the county's 60,000 high school students yesterday to produce at least one piece of writing that has flawless grammar, punctuation and spelling by the end of the school year.

Saying that he was distressed by the quality of writing being done in county schools, David told English supervisors and administrators from 22 high schools that they should "expect more of your students" and insist on improved performance in English composition.

"At times we don't expect enough of our students. They do whatever we expect of them, and if we expect mediocrity, that's what we're going to get," Davis said.

The superintendent, who once reinstituted spelling bees after learning that spelling test scores had declined, told the school officials that he intends to monitor student performance by having random samples of students' writing periodically sent to his office.

Davis said the high school students will be required to rewrite their work until it is free of grammatical, punctuation and spelling errors.

Starting next year, random samples of student's writing will be collected and screened quarterly. Samples could be business letters, research papers or original compositions, according to Jacqueline Benson, associate superintendent for instruction.

Currently, all high school students in Fairfax are required to do some form of writing. But Davis said he fears that many of them get their papers back with the errors noted by the teachers, throw the papers away and commit the same errors on their next papers. The rewriting is an attempt to make the students learn from their mistakes so they will not repeat them, he said.

Teachers at the meeting yesterday generally agreed with Davis' assessment that students' writing skills need improvement.

"I'm very interested in it," said Lanny VanAllen, an English teacher at Chantilly Secondary School and the mother of two high school-age children.

"I see the writing in my class and I see my own children's writing. They are absolutely unable to express themselves on paper," she said.

One problem, she said, is that many students lead such restricted lives that they have little to write about.

"They go home and watch television all day and night and their level of life experience doesn't go much beyond 'The Brady Bunch,'" she said.

"The problem is that they don't read very much. It's kind of frightening," said Dick Larson, head of the English department at Herndon High School. "We've got some kids who watch as much as 50 hours of television a week."

In language arts abilities, 11th grade Fairfax students tested at the 53d percentile on standardized achievement tests last fall. That means the 11th graders, the only high school students tested, did better than 52 per cent of the students tested nationwide, but not as well as 47 per cent.

However, the problem of declining composition skills is nationwide. Since 1963, scores on the verbal section of the Scholastic Aptitude Test, an examination taken by more than 1 million high school students annually, have declined steadily for the past 13 years. Colleges are complaining increasingly that they are being sent students unable to express themselves properly.

At Cornell University, for example, Robert T. Farrell, who directs remedial reading and composition programs, said. "We're getting students now who have been through four years of high school and have never had to write anything."

The Fairfax effort is but one of several in school systems throughout the Washington area to correct students' writing problems.

Just Wednesday, the Alexandria School Board directed its staff to draft a plan to re-emphasize writing skills. Arlington's public schools began an experimental expository writing program in grades 5, 8 and 12 in seven schools this year. The school system hopes to expand it to all grades in all Arlington schools next year.

In Montgomery County, all prospective English teachers are given a test in composition before being hired, and about half fall, according to a spokesman. Additionally, the Montgomery schools hire English composition aides to ease the burden of grading papers and to encourage teachers to make writing assignments more often.

In Prince George's County, all high school students are required to take a course in composition as well as literature.

Davis said his interest in composition was sparked a few weeks ago when he came across a composition of a student he happened to know personally.

"She was an outstanding student and her research was great," Davis said. "But in putting the paper together, she had done a job that was less than acceptable. It was badly organized and filled with errors.

Nevertheless, Davis said, the paper was given an A and the teacher had written, "Maryann, I know you can do much better, but I realize you had a skiing trip this weekend." The skiing trip was no excuse, Davis said, since the student had had about six weeks to prepare the paper.