As I said on another thread schools CAN be put into near total lockdown. In the security world there's no such thing as a 100% guarantee of protection, but many of the current vulnerabilities can be hardened.
Of course in order to do that we're talking about a significant increase in manpower and related security equipment for EVERY school in the country. Will the responsibility for this be at the federal level or the individual states? If it falls to the states expect a sizeable increase in taxes to pay for it. School district police departments will have to hire more officers. Those districts that don't currently have a police force will probably have to form one unless local law enforcement agrees to take on that task. If so they too will most likely have to hire more officers.
The officers or security personnel will have to control all entrances and exits from the building. Reducing the number of doors at a school, particularly a large one is not the way to go. Why, because most buildings, and I assume schools are not exempt, are built with fire codes in mind. God forbid there was a major fire and kids couldn't get out in a timely fashion because there were not enough exit points. The lawyers will have a field day.
Kids will need to be searched and all their belongings scanned for potential weapons. All others would go through the same procedure. This will include teachers, administrators, parents, and visitors - no exception. Parking lots will also be secured. Additionally kids and others assigned to the school will need to wear photographic ID, preferably around their necks secured by a lanyard at all times while on campus. There will be exceptions while actively participating in PE or sporting activities for example.
Visitors will be required to have a temporary ID with an expiration date displayed. If a kid is expelled their access to the campus will be immediately revoked. And all security and other personnel notified that they are on Do Not Admit status (DNA).
As you see with this short example things can get pretty involved. And I can assure you if such measures were implemented there will be complaints from some parents saying their little darlings are being made to feel like criminals. But if the name of the game is security, this will be the road that we as a society had better be prepared to travel.