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.
And place a minfield on the schoolyard and give each student a map to memorize. That will limit the number of potential shooters to students who go to the same school.
As for "we already have common sense gunlaws" you have the most insane and lax gunlaws of any first world country.
There is no common sense in restricting background checks when 97% of the population wants universal background checks. http://theweek.com/speedreads/756459/97-percent-americans-want-universal-background-checks-gun-buyers-67-percent-want-ban-assault-weapons
There is no common sense that you require no safety training to own a firearm when 495 people died in the US in 2016 from just accidentally discharging a firearm. https://www.aftermath.com/content/accidental-shooting-deaths-statistics
495 people! To put into perspective, it would take about 55 years for Japan to have as many gun related deaths in total (including suicide, homicide, police shootings and accidents) if Japan had the same population as USA. (They have about 30% of the US population.)
And just for the record, USA had 33,599 gun related deaths in total that year. (Although most were suicides. But the homicide rate is still astronomical.)
And there is no common sense in gun laws that produce more guns in circulation than there are people in the country. The black market definitely contributes to that, but so do your gun laws and gun culture mentality.
Your gun laws are not even close to common sense. Everyone who owns a firearm should have to go through rigorous mandatory safety training, just like you do for operating a vehicle. And you should have to renew your license on a regular basis. (Australia implemented this in their gun reform in 1996 after having had 10 mass shootings in 10 years — defined as a shooting involving five or more victims — leading up to the Port Arthur shooting. In the 20 years that followed they had one. In 2014 there was a familicide that resulted in 5 deaths. Though familicide by nature is very hard to regulate, as you can kill trusting family members with a pillow in their sleep.)
And there should be universal background checks on gun purchases, so that people with a history of violence or severe mental illness don't get their hands on guns.
Your laws even let the Orlando club shooter buy assault rifles, while he was put on a no-fly list for suspicion of terrorist group affiliations on two separate occasions.
That's grounds for preventing him from flying, but not buying guns?
Yeah, common sense my ass.