The effect of the low eligibility rate might not be noticeable now - the Department of Defense says it is meeting its recruitment goals - but that could change as the economy improves, said retired Navy Rear Adm. Jamie Barnett.

"If you can't get the people that you need, there's a potential for a decline in your readiness," said Barnett, who is part of Mission: Readiness, a coalition of retired military leaders working to raise awareness of the high ineligibility rates.

The report found that 23 percent of recent high-school graduates don't get the minimum score needed on the enlistment test to join any branch of the military. Questions are often basic, such as: "If 2 plus x equals 4, what is the value of x?" (The answer is 2.)

The military-exam results are also worrisome because the test is given to a limited pool of people: Pentagon data show that 75 percent of those ages 17 to 24 don't even qualify to take the test because they are physically unfit, have a criminal record or didn't graduate from high school.

Educators expressed dismay that so many high-school graduates are unable to pass a test of basic skills.

"It's surprising and shocking that we are still having students who are walking across the stage who really don't deserve to be and haven't earned that right," said Tim Callahan of the Professional Association of Georgia Educators, a group that represents more than 80,000 educators.

This is the first time the Army has released this test data publicly, said Amy Wilkins of the Education Trust, a Washington, D.C.-based children's advocacy group.

The study examined the scores of nearly 350,000 high-school graduates, ages 17 to 20, who took the Armed Services Vocational Aptitude Battery exam between 2004 and 2009. About half of the applicants went on to join the Army.

Recruits must score at least 31 out of 99 on the first stage of the three-hour test to get into the Army.

Marines, Air Force, Navy and Coast Guard recruits need higher scores.

Further tests determine what kind of job the recruit can do.