A COMMAND of advanced mathematics is a skill that, perhaps more than any other, opens doors for high school students on their way to college. That's especially true for those youngsters who need financial aid. In this newspaper several days ago, there was an account of Jaime Escalante, a Bolivian immigrant who teaches calculus at a high school in a Hispanic section of Los Angeles. So many of his students passed the College Board examination this year that the College Board became suspicious and insisted on retesting them. The results were much the same.

Mr. Escalante provides another reminder, in a country that frequently needs reminding, of the importance of the quality of the people who teach in high schools. Math and science require an early start. Students who have not gone a fairly long way before leaving high school are not likely to go far in these fields professionally.

Rather than increasing attention to advanced math, and the opportunities that it offers, American high schools seem to be moving, involuntarily, in the opposite direction. The reasons are the usual ones -- not enough qualified people, inadequate salaries, budget cuts.

In Maryland, to take a local example, all the states' universities and colleges produced in 1981 23 people certified to teach high school math. Last year, the number was down to a dozen. This year, there will be still fewer.

In Virginia, Alexandria runs a fine school system in which the black, Hispanic and Asian kids are three- fifths of the enrollment. The city's T. C. Williams High School has an outstanding academic program, but the schools are under pressure from the city to cut costs. A school committee put out a list this month of courses to be considered for elimination. The advanced calculus course was on the list --along, incidentally, with all of the Russian courses. In response to much well-founded criticism, the committee said, defensively, that those courses had relatively few students. That's no doubt true, but those courses also have relatively great importance for the country.

Industry, the universities and the national government all complain incessantly about the low numbers of young Americans competent in advanced mathematics -- but industry, the universities and the national government are doing precious little about it. Any improvement will take money and an enormous amount of the personal energy that goes into good classroom teaching. But with those resources, as Mr. Escalante and several dozen outstanding math teachers in this area's high schools keep showing, great things can be done.