THREE OUT of four young Americans complete 12 years of school and graduate in good order.

The strange thing about this ratio is that, after rising steadily for many years, it came to a plateau in 1968. In that year, one out of four failed to graduate and, despite immense efforts throughout American society to encourage youngsters to finish school, one out of four still fails to graduate.

There is no obvious explanation for this pattern, but it is unmistakably clear. For a century the proportion of youngsters graduating from high school rose consistently. In 1870 about 2 percent of all school children graduated. The figure reached 20 percent in the early 1920s, which meant that as many graduated from high school then as graduate from college today. The rise was continuous until 17 years ago. Then, abruptly, it stopped. Throughout the 1970s it wobbled uncertainly, falling a little and then recovering, in a narrow range. In 1983, the last year reported, it got up to 76 percent.

Since there's nothing in the structure of the population or in American educational practice to account for this persistent lack of progress, it's worth pushing speculation a little further in search of answers. Perhaps high school graduation rates illustrate a paradox concerning individual responsibility. As long as it was mainly the responsibility of the students and their families to see that they persevered through 12 years of school, graduation rates improved. But in the 1960s the balance shifted, and it became increasingly the responsibility of the school and society in general to see that everyone got through. That's also the period in which graduation rates ceased to improve. Possibly there's a relationship.

Pushing the graduation rate from 76 percent to 100 percent would be good for this country, just as pushing it from 50 percent to 76 percent was good for the country. School boards, teachers and taxpayers should firmly believe that society has a responsibility to get every child through high school. But here we come back to the paradox: evidently it's not helpful for a 17-year-old to think that the responsibility to get a diploma essentially falls on somebody else. It would follow that the most promising way to get the drop-out rate down would be to pay less attention to the drop-out rate and instead convey more forcefully to students the thought that earning a diploma is up to them.

That would represent a rather drastic departure from current policy in education. But current policy in education has left the graduation rate stuck for nearly two decades at a level that the country should not accept as sufficient.