Nearly a quarter of US high school graduates who seek to enlist in the military are turned down because they fail its entry exam, an education advocacy group said Wednesday.

Of the 350,000 graduates who met the armed forces' physical requirements from 2004 to 2009, 23 percent failed the exam, which covers English, math and reasoning, The Education Trust said in a new report.

"Just as secondary schools are failing to prepare many students for college and civilian careers, so too are they failing to prepare young men and women -- particularly young people of color -- for military service."

The study found higher rates of failure among minorities, with 39 percent of African-Americans and 29 percent of Latinos failing the exam, compared to 16 percent of white applicants.

The Pentagon administers a battery of tests to aspiring recruits, covering vocabulary, reading comprehension, science, math, logical reasoning, electronics, automobiles and mechanical skills.

A sample question reads: "If the tire of a car rotates at a constant speed of 552 times in one minute, how many times will the tire rotate in half-an-hour?"

Lower test scores on average could prevent minorities from progressing as quickly in the military, as the entry exam also determines where new recruits will be placed and can shape their future careers.

"This shatters the comfortable myth that academically underprepared students will find in the military a second-chance pathway to success," said Kati Haycock, the head of Education Trust.