the problem is definitely not law abiding citizens with legally obtained guns. It's the murderers. They do not follow the law.
More school security with heavily armed soldiers fixes this in one day...
An armed guard with a good aim is a great match against any intruder. Why not have 10 or more heavily armed and trained National Guard soldiers at each school? If the shootings continue, add more soldiers.
Add in high level ( sporting event/music concert ) gate security.
It's a money issue. Spend enough to secure the schools. Problem solved.