SPRINGFIELD -- Education officials plan to hire an independent auditor to look into why Illinois high school students posted all-time low scores on an achievement exam.

Eleventh-graders took the 2007 Prairie State Achievement Exam last spring. Statewide, reading scores dropped by more than 4 percentage points. More than 320 schools failed to meet state standards overall.

"Any time you see a drop like this, it's a concern," said Illinois State Board of Education spokesman Matt Vanover. "We want to take a close look at the whole testing process and see if we can determine if there was a problem with the test, or if this is a real decline in scores."

The two-day test consists of the ACT college entrance test and the ACT Work Keys, which focuses on real-world skills.

Officials want to know whether the test or the scoring was somehow faulty.

ACT will cooperate with an external evaluation of the Illinois scores, said spokesman Ed Colby.

Steve Cordogan, who analyzes test scores at Northwest Suburban High School District 214, said he felt relieved when he heard the state will investigate.

"We have worked so hard to improve performance," Cordogan said. "It would be very demoralizing to think that we genuinely went down (in our scores). I don't think we did."