The average scores of this year's Scholastic Aptitude Test did not fall from last year-- but neither did they rise. Nearly a million high school students take these tests every year, and they continue to be a valid indicator of academic achievement. The long slide that began in 1964 seems to have ended several years ago, but the recovery has not yet begun. That deterioration had many causes, some of them far beyond the control of the schools. But there is unquestionably a relationship between the declining SAT scores and the rapid erosion of the high schools' curriculum requirements.

Should there now be a national curriculum? As Ernest L. Boyer observes in his book, "High School," the question touches a contradiction in American attitudes. Most Americans remain deeply suspicious of uniform standards, but most Americans want uniform results. Mr. Boyer, who is the president of the Carnegie Foundation for the Advancement of Teaching, proposes a basic curriculum that covers two-thirds of the 20 or so courses that a student normally takes in four years of high school and leaves the rest to electives.

There's nothing wrong with letting students choose their own electives. The trouble starts when they choose too many electives--Family Relations, Glee Club, Creative Communications--and not much English, less math, no science and no foreign languages. A school that permits youngsters to float along with that kind of a schedule is retreating from its responsibilities. High school kids don't necessarily know what they need to know. It's the schools' job to tell them.

Establishing a curriculum begins with a decision as to what's most important, and Mr. Boyer is surely right in declaring that the first priority goes to language and literacy. He proposes a standard freshman English course emphasizing writing. Next come math and science, history and government. Last week's report to the National Science Board recommends at least three years each of math and science for all students, not only the future engineers.

School systems and, for that matter, students and their parents are already moving in that direction. The drift away from academic subjects ended in the middle 1970s and has been slowly reversing itself. Virginia increased its graduation requirements last summer, and the District of Columbia has introduced its highly useful rule requiring each student to take at least one year of a foreign language.

The high schools are now emerging from a long, unhappy period in which many of them hesitated to provide much direction to their students. Far from benefiting from a range of choices expanded endlessly and mindlessly, students often graduated with far less than the basic education to which they were entitled. The current deluge of books and reports gives valuable encouragement to the schools to keep going in a direction in which they are already moving.