I don't think any facts exist pertaining to the exact firearm used in this case, however, an ar15 rifle comes in myriad sizes, and easily breaks down in to two pieces, the barrel, and the stock, or the "upper" and the "lower". So, one example, the upper is 25 inches long, and the lower is 18 inches long....could be put in a backpack.
Then there is the ar15 pistol, which is even smaller.
I don't know the exact time of the start of the shooting, and for me, I'm always cautious as to reports of how things in these shootings transpired, no matter what the origin of the report.
I'm not convinced this murderer ever made it to class on the day of the killing. I opine he went straight to the bathroom, or elsewhere, and stayed there, easily assembled his weapon, I mean, the two parts mate together easier than tying shoes, and two pins push across the parts, one pin forward, the other rearward, where the upper and lower mate together, and you just push those pins in, as easy as snapping a jacket or coat snap.
Another item I've not heard, when the call went to the school, there is no report that the mother mentioned a gun, or suicide, or murder...not that I've seen. So, at this juncture, in defense of the school, a counselor could easily have gone looking for this young man, and had difficulty finding him, if he were hiding in a bathroom, or a dark auditorium, or whatever.
My persistent focus is on one thing, for sure...public schools must put in place procedures to prevent firearms from entering school buildings. All sorts of stuff comes in to public schools in backpacks, knives, razor knives, guns, drugs, vapes, alcohol, you name it.
Shootings on the bus, or outside areas, are a concern too, but it's difficult to assemble an ar15 on the bus and get away with it. And lastly, indeed, a kid, or adult, could wheel in to the parking lot, get out of a car, and start shooting, for sure. Shootings are a common thing in the U.S. Go to google, pick a town or city, type that in to the search bar, and then add the word "shootings", hit the go button, and you'll find all sorts of stuff. Generally speaking...the smaller the town, the fewer the shootings. There are no guarantees of safety in this world, the question is, for me, is it worth the investment to prevent the firearms from entering public schools?