The writing test was given to students last year. For the first time they used computers and were allowed to use spell-check and a thesaurus.
Even with that advantage, the agency that administers the test found just 27 percent of students were able to write essays that were well-developed and used proper language and grammar.
Test organizers say the results demonstrate that word-processing technology won't help much if students lack core skills to organize and present ideas.
Students who have access to computers at home and regularly use them for assignments are more likely to be strong writers, a national exam suggests.
Previously, young people taking the National Assessment of Educational Progress writing test had to use pencil and paper; the switch was made in line with changes in technology and a need for today's students to write across electronic formats.
Because this was the first version of the computerized test, the board cautioned against comparing the results to previous exams.
The results at both grade levels showed a continuing achievement gap between white, black, Hispanic and Asian students. At the eighth grade, Asian students had the highest average score, which was 33 points higher than black students on a 300-point scale. At the 12th grade, white students scored 27 points above black students.
There was also a gender gap, with girls scoring 20 points higher on average than boys in the eighth grade and 14 points higher in 12th grade. Those who qualified for free and reduced price lunch, a key indicator of poverty, had lower scores than those who did not; there was a 27 point difference between the two at the eighth grade.
The Associated Press contributed to this report.