There are over 100,000 public schools in the US. Each have multiple doors and many have a completely open site layout with dozens of classrooms with exterior facing doors. It would require millions of armed security which makes it neither the quickest nor most pragmatic solution. That would easily cost over $50 billion per year (1 million officers @ $50k per year). We just cut taxes. Good luck getting funding.
And let's not forget that many schools already have security and it still doesn't seem to stop them. Marjory Stoneman Douglas HS in Parkland, FL had one. Great Mills HS in Maryland had one and he shot an innocent student.
And this one, Santa Fe HS in Texas, had a school officer...the suspect shot him.
You're exaggerating to the point of strawmanning especially with your numbers. Limit the entrances in a school to 1 or 2 and equip them with metal detectors and use a few young/healthy ex-military with more than side-arms with metal detectors. Make every school a hard target.
Banning guns is off the table. We already have common sense gun laws. There's not alot of practical solutions or easy solutions. We're gonna have to refine what's there already.