While there are obviously problems, I think there is also an issue of perspective, especially for our non American friends.
First things first, while guns laws differ from state to state, nowhere in the USA can a person just walk into the local supermarket and legally purchase a gun.
Federal level background checks are required, and most states also require some form of licensure. If a person has already gone through the states process and had the Federal background check completed, a person can walk into a gun store and walk out with a gun in about an hour.
Automatic weapons have been outlawed from civilian ownership in the United States for a hundred years. An automatic weapon is one in which it will keep firing so long as the trigger is held down. A machine gun.
A semi-automatic weapon is one in which it will fire one shot each time the trigger is pulled. The vast majority of all firearms across the world are semi-automatic. The AR-15, while commonly used and scary looking, is not a particularly powerful or accurate weapon. There is nothing particularly special about them as far as guns go. They just look intimidating and are cheap. A good hunting rifle is far more powerful, shoots further, and is more accurate.
In the context of 330 million people and 500 million guns, almost all of the gun violence is isolated to a few geographically tiny areas, most of which have already have extremely strict gun restrictions. The overwhelming majority of geographic America is largely devoid of any gun violence, especially in the more rural areas where legal gun ownership is the highest. Hell, those small rural towns can go decades without any weapon related crime, where the worst they see is a few bar fights, kids breaking into a house or spray painting a shop front, or bubba getting drunk and slapping his wife around. Not good for sure, but hardly the war zones as is often suggested.
The quoted statistics for what qualifies as mass shootings include any incident in which two or more people are injured or killed, so included in the totals, the majority of mass shootings are made of the typical weekend shootings in the few high crime areas such as select neighborhoods in Chicago, New York, Baltimore.
The particular types of mass shootings that make world headlines, especially those occurring in schools, while tragic with shocking numbers, make up a very small portion of the total gun violence, which again, is extremely isolated to a few areas. What these mass shooting do however is they affect the psyche in the same way that any other terrorist attack does around the world. They happen in places most people can personally relate to such as schools, churches, restaurants, workplaces. It gets people thinking...if it could happen there, it could happen here as well. That is the whole point behind effective terrorism. Not necessarily to rack up body counts, but to make people fear their day to day lives and activities.
In the end, the vast majority of school kids have, and will continue to go through their time in school without ever personally experiencing anything like this. They are far more likely to experience a classmate getting dunk at a party and dying in a car wreck with a few other classmates.