WASHINGTON — Suspension rates dropped for many of the nation’s school districts, but U.S. students still lost about 18 million days of instruction to out-of-school punishments in the 2011-2012 school year, according to research released Monday.

Researchers at UCLA compared detailed data for every U.S. school district, presenting a mixed picture of discipline at a time of increased focus on the issue nationally.

They identified school systems in Missouri, Ohio, Michigan and Pennsylvania that they said showed “alarming” suspension rates of 20 percent or higher for elementary school children. But they also pointed to 28 school systems in 17 states that had marked declines in suspension rates from 2009-2010 to 2011-2012, the most recent national data available. They found more than half of the country’s school districts had relatively low rates of out-of-school punishment.

“There are some large districts that have made some dramatic reductions in their suspensions and reduced the racial gap as well,” said researcher Daniel Losen, director of the Center for Civil Rights Remedies, which is part of the Civil Rights Project at UCLA. The group’s report is called, “Are We Closing the School Discipline Gap?”

Even with some notable improvements, national suspension rates have not changed in a meaningful way and racial gaps persist, Losen said. Across all grades, 16 percent of black students were suspended in 2011-2012, compared with 7 percent of Hispanic students and 5 percent of white students.

The report comes at a time of heightened concern about out-of-school suspensions, which researchers have linked to greater risks of academic failure, dropping out of high school and involvement in the juvenile justice system.

The federal government issued discipline guidelines last year amid an effort to keep more students in class, reduce racial disparities and avoid unnecessary suspensions. Many experts have urged schools to use alternatives to suspension when possible.

“We conclude that our nation cannot close the achievement gap if we ignore the discipline gap,” the UCLA report said.

The researchers found racial disparities were pervasive. But many school districts showed relatively low suspension rates for all racial and ethnic groups.

The researchers urged education leaders to examine the data for lessons about best practices, to put more resources into training teachers and school leaders and to use school climate as an accountability measure.

The report cited suspension rates as high as 50 to 60 percent for secondary students in individual school districts in Illinois, Mississippi, Michigan and Arizona. “It’s really inexcusable,” Losen said. “These are school districts that are shouting out for help or intervention of some sort.”