Mental Health screening in high school, maybe middle school as well. Help for those for whom the screening suggests.
No (legal) gun purchases for those with specific mental health concerns.
Locks on gun cabinets, trigger locks that only the owner can bypass.
Metal detectors in all public places.
Social Change to deal with the violence in our culture, though that's the hardest of all.
The discussion has to happen, and has to happen soon. Mental health professionals, Gun (or any weapon) rights advocates and safety advocates, law enforcement, media (violent programs and games?), clergy,...let the hunters hunt, let the target shooters shoot targets, let the self defenders defend themselves, let the collectors collect, let the 2nd Amendment stand...but somehow the less legitimate uses need to be dealt with.
No system is perfect, someone who is determined will find a way, yes, it could be a car running through the playground or doing something to the ketchup, but that's no reason to not start a serious discussion about how to reduce the violence.