There are a lot of factors. The entire social atmosphere a child experiences, influenced for example by economic stress on parents and teachers, and how much the system enables teachers to do a good job.

But yeah guns definitly play a role. Most school shooters take their guns either from parents or other social contacts, enabled by a high density of guns per capita and a big gun market with poor regulation. The Columbine shooters got theirs from a friend who purchased them at a gun show.

A few school massacres happened here in Germany as well. The murderers generally got their weapons from their parents. Those attacks without firearms ended up much less lethal. The attacker of Winneden in 2009 for example had a father who owned 15 guns. He had 14 of them locked up but one set aside for emergencies, which ended up being the gun used for the shooting that killed 15. Meanwhile in Ansbach 2009 the perpetrator had no access to firearms. He came with an axe, knives, and two molotovs. None died, two were severely injured, seven lightly. Terrible enough that it happened, but a much preferrable outcome.