Teenagers’ texting, tweeting and posting on Facebook hasn’t improved their writing, even when students have laptops with a spell-check program.

Nearly three-quarters of the eighth- and 12th-graders failed to achieve proficiency on a national writing test, according to a U.S. government report released Friday. For the first time, the exam let students use a computer, rather than pencil and paper.

Most students’ writing “falls far short of the well-organized, well-developed prose that connects with those they are trying to reach,” Susan Pimentel, a member of the U.S. Education Department board overseeing the test, said in a statement.

The 2011 test, known as the Nation’s Report Card, adds to concern about American schoolchildren’s knowledge of math and science relative to other countries.

The new version of the test, officially called the National Assessment of Educational Progress, offered students the tools of modern writing: a laptop with a word-processing program, including spell-check, cutting and pasting and other editing functions, as well as a thesaurus.

More than 24,000 eighth graders and 28,000 12th graders took the exam. On the writing exam, 24 percent of students were considered proficient in writing and 3 percent ranked as advanced.

The 2011 results can’t be compared with the past pencil-and-paper exams. In 2007, the last time the government assessed writing, scores had increased from five years before, though most students also had poor writing skills.

On the latest report card, students who wrote more often at home did better on the test. So did those who made use of computerized tools during the exam to revise their work or find words on the thesaurus. By contrast, those who relied heavily on spell-check scored lower.

Mirroring demographic results on other tests, Asian students outperformed other ethnic groups in eighth grade. In 12th grade, white, Asian and multiracial students performed comparably. Whites did better than blacks and Hispanics. Poor students lagged richer ones. Private and Catholic schools scored higher than public schools.

Girls beat boys in writing scores by a higher margin than for any other subject. On questionnaires, girls said they wrote more and were more likely to call it a favorite activity.

The Education Department judged writing based on organization, level of detail and variety of sentence structure. The students wrote for 30 minutes.

In one eighth-grade assignment, students imagined they were stranded on an island. A weak response featured the following sentence: “There is five guys and five girls, the girls will get to sleep inside the plane so they don’t get to cold or scared.”

One of the stronger passages built suspense, conjuring a threat from dinosaurs: “We slowly trudged through the dense sand back to our boat, which was now in sight. But it seemed that time stopped and the next thing I saw was a gigantic foot on top of our ruined boat. A dinosaur’s face 20 feet above leered down at us and growled.”