During the current school year, The Washington Post is examining the challenge posed to schools and society by the increase in the number of "children at risk."

In greater proportion than ever before, young students are poor, non-English-speaking, and have teen-aged mothers, unstable families or ethnic backgrounds statistically linked to lower academic performance. The reasons are several: Immigration is up; the increase in divorce means that 60 percent of today's 3-year-olds will live in single-parent homes before they turn 18; birthrates among poor and minority families are higher than those among middle-class whites, and the proportion of children in poverty has risen from 15 to 20 percent since 1970.

As a result, higher proportions are likely to drop out, become pregnant as teen-agers or fall victim to crime, drugs or chronic unemployment.

At the same time that the proportion of children "at risk" of failure is increasing, the school-age population has declined, from 53 million in 1970 to 45 million last year.

This combination of statistics carries serious implications for society as the baby-boom generation ages and today's young people join the economic mainstream. With high school graduates declining in number, employers will be forced to hire from a smaller pool of applicants. No longer will they be able to bypass the young people who are least prepared.

The conclusion, say educators, is that schools must succeed with a higher proportion of students than ever before, although student bodies are now more disadvantaged.