The white sheriff’s deputy who grabbed a black student’s chair, flipped her over and dragged her across the floor on Monday at Spring Valley High School in Columbia, South Carolina, prompting an FBI civil rights investigation, worked as a school resource officer. SROs, law enforcement officers who are specially trained to work in schools, are a common sight in public schools, but their work is seldom scrutinized — and the Spring Valley High incident is focusing attention on the danger that their presence could have a disparate impact on black students. A 2008 study by the ACLU of SROs in three Connecticut towns found that black students were disproportionately likely to be arrested or reported to the police by an SRO.

How common are these officers? According to a National Center for Education Statistics survey of 1,600 public schools nationwide, 30 percent had a school resource officer on the premises in the 2013-14 school year. Eleven percent of public schools reported that they had a law enforcement professional on their campus who was not trained specifically as an SRO.

SROs are present at almost half of high schools and middle schools (49 percent and 46 percent, respectively), while 18 percent of elementary schools have an SRO on duty. Large schools like Spring Valley High (enrollment 2,010) are the most likely to have an SRO working in the halls; 73 percent of schools with 1,000 or more students employed one.

The federal government has encouraged schools to bring cops into the classroom. SROs are intended to deter shootings and other violence in schools, while also becoming a positive presence in students’ lives and increasing trust in law enforcement. In 2013, the Department of Justice spent $45 million to create 356 new SRO positions nationwide, which represented a third of all new law enforcement positions that the department created through its Office of Community Oriented Policing Services (COPS) that year. The office’s grant-making process gives additional consideration for funding to applicants who plan to hire SROs.

Although COPS encourages grant applicants to add SROs as part of their community policing efforts, the officers are seldom evaluated to see whether they change students’ relationships with law enforcement for the better. A 2005 Justice Department study of 19 new SRO programs found that “very few … conducted useful and valid assessments of their programs.” When schools do any sort of evaluation, they are prone to track things like weapons confiscated, fights broken up, etc., but these are tricky ways to measure success. If, say, an SRO disciplines more students for any particular infraction, does that mean more infractions are taking place or just that the officer is recording more of them? Are students behaving worse than they were before? Do crackdowns deter future rule-breaking or enhance student safety?

The study recommended that schools include metrics like graduation rates in their evaluation of SRO programs, not just incident reports. The Justice Department also suggested that schools include surveys of student attitudes about their SROs in assessments.

One 2005 Justice Department study of student attitudes surveyed 907 students across four school districts about their relationships with the SRO at their schools. Overall, 74 percent of students had a positive opinion of the SRO on their campus, but opinions varied considerably among schools, with a low of 43 percent feeling positively and a high of 99 percent. Although students might approve of the SRO in the abstract, only 64 percent felt comfortable reporting a crime to their school’s officer, and only 50 percent said they’d be comfortable approaching the SRO with any sort of problem.

For many students in the study, the officer on campus was more a uniform than an individual, even though an SRO is meant to build relationships. Thirty-eight percent of students didn’t know the name of their school’s SRO, and over half (55 percent) had never spoken to him or her.