WASHINGTON — Each year, more than 3.25 million public school students nationwide are suspended at least once. But black students are suspended three times as often as white students, according to a report released Wednesday by the National Education Policy Center, a University of Colorado education research organization.

The number of suspensions has risen significantly over the past 40 years.

“It’s predominately minor infractions that don’t require law enforcement,” said Daniel Losen, the author of the report, which was put together by compiling older studies and school records.

The report found that 6 percent of African-American and 3 percent of white students were suspended in the 1972-1973 school year. Fifteen percent of African-American and 5 percent of white students were suspended in 2006-2007, the latest year for which data was available.

Being suspended can have a dramatic effect on students’ development, the report said.

“One of the leading indicators of whether students drop out or wind up incarcerated is whether they were suspended in middle school or high school,” Losen said at a news conference Wednesday to release the report, which gathered state suspension data through Freedom of Information Act requests.

In one example, North Carolina had one of the most pronounced disparities between blacks and whites for minor infractions, according to the data. In 2010, 43 percent of its first-time offenders suspended for displays of affection were African-Americans, while14 percent were white students.

Of those suspended in North Carolina for using cellphones in school, 32 percent were black and 14 percent white.

Nationally, the disparity in suspension rates continued for students with disabilities. In the 2007-2008 school year, 16.6 percent of African-American students with disabilities were suspended compared with 6.7 percent of white students with disabilities, according to the report.

The U.S. Department of Education is expected to address school discipline when it releases a major report on education and civil rights issues this fall.

Many schools have zero-tolerance policies that suspend a student after a first offense.

“There are better ways to solve these problems than to just send people home,” said Jonathan Brice, a school support network officer for Baltimore schools, who spoke at the news conference.

Four years ago, Brice reduced suspensions for minor offenses in the Baltimore schools. Since then, graduation rates have risen from 51 percent to 71 percent and the dropout rate has fallen from 10 percent to 4 percent, he said.

Wanda Parker, a mother from Greenville, Miss., saw firsthand the effects of a zero-tolerance policy when her son, James, was removed from his school. His school believed his iPod was a cellphone and placed him in an alternative school setting even though he had no cellphone, she said.

“They dehumanize the schools and make them feel more like a prison than a second home,” Parker, who is African-American, said at the news conference.

Several states are trying to decrease the number of suspension-prone schools. Connecticut law requires almost every school code infraction to be punished with in-school suspension rather than out-of-school suspension.

Maryland passed a law in 2004 requiring any elementary school that reaches a suspension rate of 10 percent of its enrollment to engage in a behavioral intervention program.

“This is a myth busted that we have to kick out the bad kids so the good kids can learn,” Losen said.