Black students are dropping out of high school less, falling behind less and going to college more than a decade ago, the Census Bureau reported yesterday, but they still haven't caught up with whites.

"We are improving but the gap is still there," said Carol Gibson, director of education for the National Urban League in New York.

The dropout rate is determined by surveying youths just above high-school age to see whether they graduated. Among blacks the proportion of dropouts declined from 35 percent to 25 percent over the past decade. Although a substantial improvement, the figure was still far higher than the 14 percent rate for whites.

The figures also showed significant improvement among blacks in keeping up with the proper grade for their age. In 1967, 18 percent of all black students aged 16 and 17 were at least two years behind their proper grade. By 1977, this had fallen to 10 percent; for whites it was 3 percent.

Overall, the share of blacks 18 to 24 with a high-school diploma jumped from 56 percent to 68 percent from 1970 to '77; the 1977 white figure was 82 percent.

Once they do get high-school diplomas blacks enroll in college in about the same proportion as whites -- one of every three hish-school graduates.

They now account for 11 percent of all college students, just slightly below their 13 percent overall proportion in the population, and for 6 percent of all graduate students.

Gibson said that, while there has been progress, blacks "clearly have not closed the gap with white America."

She said overoptimism might result from the figures on college enrollments. "More are going on to post-secondary experiences," she said, "but not necessarily in four-year colleges." She said many merely go to local two-year colleges and never get full four-year training and degrees.

Larry Suter, chief of the education and social stratification branch of the Census Bureau, said, however, that "the difference is not as big as you think." In 1974, he said, 23 percent of white college students were in two-year institutions and 29 percent of blacks -- not such a big gap.