And with no way to call for help, attacking beforehand would have likely led to more deaths, including the boys, teachers, and infants, as well as the girls.
We all want to think when we hear about things like this that somehow it could been prevented. The idea that a person who is determined to murder innocents and does not fear death can attack successfully is quite chilling. But from what has been released so far it doesn't appear that anyone could have stop this slaughter except the killer himself.
Every situation is different. But I don't see the harm in having dialogue about what one might do if caught in a future situation.