Impossible?

The Arizona Wildcat, the university’s newspaper, ran an AP story yesterday about a measure in Texas that would legalize concealed carry on college campuses by permit holders.
Evidently due to space limitiations, the Wildcat cut off the article after the 8th paragraph, which ended in “Texas campuses are gun-free zones.” Of course, the part of the article that was snipped had nearly all the quotes and text explaining why one would want to carry on campus, or that people with permits can carry just about everywhere else and that the campus border is just an imaginary line. (( Bias? What bias? ))
One quote from Woods, a student whose girlfriend was killed in the Virginia Tech shooting, stood out to me:

“Then I learned pretty fast that wouldn’t solve anything,” said Woods, who is now a graduate student at UT. “The idea that somebody could stop a school shooting with a gun is impossible. It’s reactive, not preventative.”

Impossible? Impossible? To quote Inigo Montoya, “You keep using that word. I don’t think it means what you think it means.” Last time I checked, cops carry guns, and they’re able to put an end to murderous violence pretty effectively with those guns. There’s a lot of private citizens who carry guns, and numerous stories about how those people use their guns to stop violent crime.
One might not be able to prevent a school shooting, but one certainly would have the potential to stop it.
I know that if a person bent on murder came barging into my classroom, I’d prefer a means of resisting them that’s more effective than a thrown textbook.