That figure masks large gaps between student groups — with 43 percent of Asians, but only 5 percent of African Americans — demonstrating college readiness in all four subjects.
The ACT is a competitor of the SAT and is now the most popular college entrance exam in the country, with about 1.8 million graduating seniors taking the test this year. That number accounts for about 54 percent of the nation's graduating seniors and was an increase from 1.67 million in 2012.
Overall performance on the ACT has remained virtually unchanged since 2009, with the average score falling slightly this year, from 21.1 to 20.9 out of a possible 36 points. The stagnation raises questions about how well schools are preparing students for future success.
"This report demonstrates that we must be honest about our students' performance and implement higher standards if we're serious about improving educational outcomes," Education Secretary Arne Duncan said in a statement.
Duncan is a supporter of the new Common Core State Standards, strict academic benchmarks that have been fully adopted in 45 states and Washington to help ensure that students are prepared for life after high school.
The increasing number and diversity of test-takers might be one reason for the slight decline in scores this year, as it is not uncommon for average performance to fall as the pool of test-takers grows.
Critics of federal education policy said the continued poor showing on the ACT is proof that the nation's focus on measuring school performance with standardized tests — which has intensified since the No Child Left Behind law was passed in 2002 — has not led to improvements.
"The overwhelming evidence that college preparation is not improving — even when measured by test results — shows that politicians' fixation on high-stakes standardized exams to boost student performance is a failed strategy," said Bob Schaeffer of the National Center for Fair & Open Testing.
ACT officials said they set "college ready" benchmarks that reflect the minimum scores students need to earn to have a 75 percent chance of earning a C or better in a typical first-year college course.
Two-thirds of test-takers met the college-ready standard for English. In math and reading, 44 percent did so. Science performance was worst, with 36 percent of students deemed college-ready. Only 26 percent of students who took the ACT reached the college-ready benchmark in all four subjects.
In Maryland, 21 percent of high school seniors took the ACT this year, and participation was somewhat higher in Virginia (26 percent) and Washington (38 percent).
Students in Virginia and Maryland fared better than the national average, with higher composite scores and a higher proportion of students meeting college-ready benchmarks. More than one-third of students in public and private schools in Virginia and Maryland — and 29 percent of Washington students — scored high enough to be considered college-ready in all subjects.
Students' scores increased slightly in all three jurisdictions, although Washington continued to trail the national average.
"Maryland did have some good success here," said William Reinhard, a spokesman for the Maryland State Department of Education. "What's difficult to read from the data is who those students are. We don't have a good breakdown yet but the trend is positive."
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