Maybe a clue will be when your wife calls and is begging for someone to come in and save her and the police are cuffing people to keep them from going in, while they are staying outside themselves?
There is no real answer, because every situation will be different. Not enough caution could make it worse, too much and we see what happens there too. But in this particular situation, gunman in the school, shots fired, known to be in a classroom full of kids, it's not really a choice of escalation at that point, but they made it one anyway, and I hope they all burn for it.
Active shooter situations are way too fluid to have a set answer or plan, generally the strategy has evolved to simply engage them at first opportunity. What to do if police are not doing that? The problem is if you as a citizen try to engage them, even if you are among the targeted victims the chances are very high you will be shot by cops even if you succeed and end it for the perpetrator. And there is less repercussion for that than even in the cases they held back.