More than 500 kids have been killed at or near a school in the last 24 years, and federal researchers say the campus death toll remains at a level that is "unacceptably high."

A new analysis from the Centers for Disease Control and Prevention shows that rates of multiple-victim school killings – nearly all of them shootings – rose significantly from July 2009 to June 2018, following 15 years of decline. Single-victim homicide rates, meanwhile, remained relatively level during that time frame.

"Although school-associated youth homicides account for [less than 2 percent] of all youth homicides, they are devastating for families, schools and entire communities," CDC researchers said. Understanding the trends and patterns in these incidents can help prevent future violence, they added.

CDC researchers analyzed federal data on homicides occurring on or around campus or at a school event from July 1994 to June 2016, and used media reports to add data from July 2016 to June 2018 on school-related homicides where there was more than one victim.

In all, researchers tallied 514 people ages 5 to 18 who were killed in 431 homicides during the time period studied.

Between July 1994 and June 2016, 90 school-aged kids were killed in 30 multiple-victim homicides associated with schools, the study found. During the next two years alone, however, another 31 youth were killed in eight multiple-victim homicides.

Across the 24-year period studied, guns were used in 95 percent of the multiple-victim homicides, and a fifth of the killers died by suicide after. Their victims were relatively evenly split among boys and girls and older and younger kids.

Among the more recent incidents: The mass shooting at Marjory Stoneman Douglas High School in Parkland, Florida, that left 17 people, including 14 students, dead in February 2018 and resulted in a wave of youth activism on gun control.

Since the Parkland shooting, the nonprofit Everytown for Gun Safety has tallied 36 gun attacks resulting in injury or death at elementary, middle and high schools and college campuses across the country.

Most school-related killings involved a single victim, CDC researchers found, and those patterns were generally stable over time. Youths living in urban areas, boys, non-whites and older teenagers were more likely to be victims, and about 6 in 10 single-victim homicides in or around schools involved a gun.

"The frequent connections with gang activity and interpersonal disputes suggest that school-associated homicides might often be a reflection of broader community-wide risks," researchers said.

Ending violence among young people requires efforts to reduce risk for kids who have been violent in the past, helping young people learn how to cope with their problems and promoting "connections between youths and caring adults," CDC researchers said.