The goal is that it hopefully breaks down barriers between kids and law enforcement officers.

http://www.pbs.org/newshour/updates/school-resource-officers/ is a fun article:

Editor’s note: After a video of a South Carolina school resource officer (SRO) forcefully removing a student from her desk went viral, a debate about the presence of law enforcement officers in schools lit up on social media. The officer, Deputy Ben Fields, was fired and federal authorities have since opened a civil rights investigation into the incident.

In response, school resource officer Justin Schlottman of Cedar Crest High School in Pennsylvania shares his thoughts on why he became an SRO and how he seeks to build a safe and trusting school environment.

https://www.newamerica.org/education-policy/edcentral/school-resource-officers/ said:

Although law enforcement has traditionally had some involvement with schools (think Officer Friendly, McGruff the Crime Dog, and stranger danger), it has not been until the last couple of decades that officers have been assigned to work in and patrol schools. During this period of increased police presence and security measures, violence at schools has generally declined.

However, it is difficult to say definitively whether increased security measures, including the presence of SROs, caused the decrease in school crime, or if it is simply following an overall trend of decreasing crime in the U.S. And considering the hundreds of millions of federal dollars that have funded school resource officer programs, there is a surprising lack of comprehensive research on the efficacy of SROs for preventing crime in schools.

So, if the officer is there in the school, they might as well be ready to respond to an armed shooting incident, however, as the latter article points out:

While it is possible that an armed police officer may serve as a deterrent to a potential shooter or may encourage students and/or teachers to report suspicious behavior, there is little empirical evidence to that effect. In fact, Columbine High School had an armed police officer on campus and another one nearby in 1999 when two students shot and killed 13 people.

Now, the number of people who were killed might have been much higher than 13 if an armed officer wasn't there. However:

Critics of SROs argue that the cost to students, especially minority, low-income, and students with disabilities, is too high. In some cases, disciplinary actions that once would have been handled by teachers, a trip to the principal’s office, or detention are resulting in arrests and entrance into the criminal justice system—a path often called the “School-to-Prison Pipeline.”

So it may or may not be a nice idea, and apparently nobody seems to be entirely certain whether or not it's actually working.