There are already class 3 requirements on real war weaponry. AR-15s aren't. Just because they sort of look like M4s doesn't mean they are. Take a Porsche and put a beetle engine in it, it might look like a Porsche but it's not.
Still, I would make it a crime to be within 500 yards of a school with a gun period. They do it with pedophiles and drugs, do it with guns.
Secondly, regulate the ammo. Forget clips. Make laws that you cannot have more than 12 bullets in public unless you're at a licensed range.
Thirdly, require all guns to be registered.
Fourth, you can constitutionally own a gun but you can't take it out of your house or property without a permit. Require a proper course to obtain a permit.
Fifth, if your gun is used in a crime and you did not have it properly secured, you're culpable.
There is a start for you.