ONLY 21 states require students to attend high school until they graduate or turn 18. The proposal President Obama announced on Tuesday night in his State of the Union address — to make such attendance compulsory in every state — is a step in the right direction, but it would not go far enough to reduce a dropout rate that imposes a heavy cost on the entire economy, not just on those who fail to obtain a diploma.

In 1970, the United States had the world’s highest rate of high school and college graduation. Today, according to the Organization for Economic Cooperation and Development, we’ve slipped to No. 21 in high school completion and No. 15 in college completion, as other countries surpassed us in the quality of their primary and secondary education.

Only 7 of 10 ninth graders today will get high school diplomas. A decade after the No Child Left Behind law mandated efforts to reduce the racial gap, about 80 percent of white and Asian students graduate from high school, compared with only 55 percent of blacks and Hispanics.

Like President Obama, many reformers focus their dropout prevention efforts on high schoolers; replacing large high schools with smaller learning communities where poor students can get individualized instruction from dedicated teachers has been shown to be effective. Rigorous evidence gathered over decades suggests that some of the most promising approaches need to start even earlier: preschool for 3- and 4-year-olds, who are fed and taught in small groups, followed up with home visits by teachers and with group meetings of parents; reducing class size in the early grades; and increasing teacher salaries from kindergarten through 12th grade.