Sweden and Canada have pretty high rates of gun ownership and don't have this problem. That said American school shootings are not as common as they are made out to be, there has been a lot of statistical fudging to make it look so much worse than it is.
What all three countries do have are problems with gangs and they're only getting worse as poverty drives people to crime. America has it worse because it has more poverty, but we will all catch up soon enough.