EVERYONE knows someone who blames the SAT for getting him rejected from the school of his choice, but when the same thing happens on the elementary school level, word doesn't travel quite so fast. Some private high schools and junior high schools screen applicants with the Secondary School Aptitude Test (SSAT) or the Educational Records Bureau (ERB), both of these bear enough resemblance to the rest of the standardized-test corpus for Stanley Kaplan to start prepping eleven year-olds for them on a regular basis. The most trustful ground for testing pre-high-schoolers and young children, however, has been the one which lends itself most easily to misuse and misinterpretation--the basic competency movement of recent years.

Laudable in themselves and truly constructive in many communities, basic competency tests attempt to determine what students in all grades are learning by administering SAT-like batteries at least once a year. Massachusetts competency rules, actively implemented for the first time in 1980-81, allow communities to construct their own tests and set their own passing scores, though pre-designed and prescaled tests are also available. The Cambridge public school system this month began analyzing the results of its second battery of testsone set for third, sixth and eight-graders and another for high schoolers-and comparing them to the first. And thus far, reactions among school officials indicate that Cantabridgian children will circumvent the basic competency program's main flaw-that the tests can be used to rank children as to intelligence too early and for good.

"The real value of this or any testing data, "Richard Wood ward, assistant superintendent of schools, said last month." is to learn what we can do "Cambridge school principals and officials have noted that students throughout the system tend to meet the city's minimum standards in math, but fall far below in reading and writing. With this nationally noticed deficiency in students verbal abilities comes a more unusual figure, and one Cambridge may be able to do some thing about System wide fewer sixth graders met the competencies than either third or eights graders. And in all schools and all grades, percentages of students meeting the minimum were lower in spring 1982 than in spring 1981.

THE CRUCIAL QUESTIOIN is, of course, what the school system decides to do about students who don't meet the competency level for their grade. If, as officials indicate, they treat deficiencies as the fault of the school or of some particular hitch in the curriculum, the use of standardized tests can do nothing but good. If the scores raise what Mary Lou McGrath, director of elementary education, calls "people's level of awareness of accountability" to students, then results like the reading and writing weakness and the sixth grade dip are useful warning signals about where improvement is necessary.

But such improvement takes time, effort and of course budget, and officials note that the best results in the past have come from long-term aids like better teacher training. An ever present temptation is to get rid of the problem on the surface by "holding back" students till they have attained competency for the grade level. New York City ran into just this problem in 1980 when it created an ironclad regulation for the city's overcrowded, almost completely ineffectual seventh grades: Students could not go on to eighth until they had achieved certain flat scores on the city "idiot tests."

On the one hand, that flat rule regulations raised all the usual hue and cry about standardized tests' imprecision and inherent unfairness; the rule dis-criminatex against inner-city students whose vocabularies were different, against students from poorer families, and so forth. But the more pressing problem turned out to be one of mere practicality. So many students failed to pass seventh grade that the already crowded facilities became almost useless; and a substantial number of students, though tested several years in a row, failed to pass. The presence of 17- and 18-year-old seventh-graders was a dramatic condemnation of a school system that couldn't teach the competencies, and so punished the students for not learning what was never sufficiently taught.

Such unfair measures are easy to implement, but a more insidious use of standardized scores for young children does damage which is far less visible. The existence of widespread and continuous testing makes easier a process many school nationwide have been practicing for some time-"weeding out" the very talented and the very slow for "magnet" or "alternative" tracked program, which then remain totally separate from the larger group.

"Tracked" programs, of course, do wonders for those children initially chosen, who can then benefit from special resources, special attention and smaller numbers to cruise up the college-bound route. The striking success of "UP ward Bound" programs around Cambridge, which target talented junior high schoolers and send an impressive proportion to college, shows just how much good some extra attention can do. But in choosing students on the basis of test scores, any such program excludes a few students who deserve that extra chance, and almost every recent survey of high schools nationwide has found a number of such over-rigid tracking programs.

So far, all the signs are bright that Cambridge public school officials will resist the lure of all the new numbers they have to play with. One schools committee member apparently fell into the trap of looking at the scores as a indication of primarily students, not schools; "With a few exceptions," she said, "they're horrendous."

But most comments on the year's results emphasize themes like "increased accountability," better teacher training, and extra support for low-income, minority and bilingual students. Most important, some members have mentioned the spring budget talks as a time to push for these goals. Standardized basic competency tests can continue to serve as instruments for such needed progress-as long as teachers and administrators keep in mind who it is they are testing.