None of your laws would have prevented Cruz from shooting up the school. Law enforcement, on the other hand, would have prevented it entirely.
The first sentence is prima facie wrong as worded; and it misses the point, which I'll repeat. We can not know the future and certainly anecdote can at best give us only a glimmer of what might be, or indeed, what might have been. But we can rely on probability. The gun control debate should center on probability; not on some non-germane slogan or non-sense about the Second Amendment that the NRA invariably tries to inject. We know why they do it. They do it because they are paid lobbyists for the gun manufacturing industry, and their trite appeal to emotion has succeeded despite its lack of logic. The Parkman kids' arguments are based on probability , whether they realize it or not. Therein lies the strength of their arguments. And they have their sights fixed on the Bullseye! -- those congressmen who have tossed logic out the window to get NRA campaign funds. They will succeed where others have failed.
The second sentence is a statement that is incorrect as written. If it was rephrased to read: "Had the circumstances been different, law enforcement might have prevented the event," then the sentence would be correct. There are no proposals, with any traction, regarding changes in law enforcement that would make your second sentence correct as worded. You can reduce the severity of a mass shooting at a school by having a vigilant, armed, on duty officer on site at every school. But you can not prevent entirely future similar tragedies. Regardless of these simple statements of the obvious, as long as the arguments are phrased in incorrect absolutes, replete with anecdotes, nothing of substance is contributed.
Another point is being missed entirely by the simple minded approach suggested in your second sentence. I'm referring here to schemes such as arming teachers or having an armed guard always on duty. In light of the broader problem, these piecemeal ideas are absurd. Has everyone forgot that schools have no monopoly on mass shootings?
The Parkman kids on the other hand have applied logic and probability to obtain the most practical, effective and broadly applicable answer to the problem. What they want has a real chance of gaining enough political traction to become law. Their proposed remedies will reduce the probability of mass shootings everywhere, not just in schools!