The high school class of 2007 posted a modest increase on the ACT college entrance exam, extending the test's upward scoring trend and showing improved levels of preparation for college.

Still, the results highlight the persistent gap between the preparation levels of high school graduates and the skills they need to do well in college. Only 23 percent of test-takers met a benchmark score that indicates readiness in a range of introductory, core college courses.

The national average ACT composite score rose from 21.1 last year to 21.2 on a scale of 1 to 36 extending its recent pattern of slight but noticeable increases.

The ACT says one-year trends are not necessarily meaningful, but that the average scoring increase of 0.4 points since 2003 is significant, considering 1.3 million of this year's high school graduates took the test.

"Surely we have a lot of work ahead to be sure all students graduate from high schools with the skills they need to succeed at the next level, but we do seem to be making some encouraging progress," said Richard Ferguson, CEO and chairman of ACT, the Iowa City, Iowa-based nonprofit that owns the exam.

The ACT believes the gains reflect genuine improvements in learning, but interpreting the results is complicated by the changing composition of the pool of people who take the ACT.

In some states, growth is coming from relatively low scorers who may not have taken any standardized tests in the past, and who weigh down average scores. But in other states, such as Connecticut, New Jersey and California, much of the growth appears to be coming from a different group: high achievers who also took the SAT and are increasingly trying the ACT, too, in hopes of impressing selective colleges.

The ACT doesn't know how many students take both tests, but overall, 42 percent of this year's high school graduates took the ACT, up from 40 percent last year.