Absolutely. If just some teachers and staff comfortable with firearms and trained in their use were in schools we would not see these go down, because these mad men are cowards.
We might find out, someone on the staff did have a firearm. That school had 4,000 students they said on Fox; at this point, it looks like 17 have died (perhaps a teacher or two is in that count). One can see how it would be difficult to be fully secured.
Something about this attack is real disturbing, even with strict gun laws, these kinds of incidences are one offs, yes, maybe metal detectors would have been needed. Even if the school had very good security, a gunman can always look for another soft target.