No teacher wants to harm a student. I was on the fence about this, it feels counter to why I am there, to teach young people.
It's stange at first because is a very complex issue for a teacher, one that is different than say law enforcement who would have no connection to the shooter.
Despite the press, school shootings are rare. Also, you seem focused on a current student doing the shooting. Given your small size and how well you know your students and circumstances, it might be more likely that a former delinquent student (who has graduated from theft to murder) or random psycho would be the shooter. In any case, if you care about your students, then take comfort that you are saving their lives should one of the students go rogue.
If you volunteer, are you OBLIGATED to act, or is it entirely your decision as to whether you draw your weapon and engage? Can you choose to defend yourself and not others (e.g., down the hall)? LTC holders have to decide if they want to confront a shooter next door or use the pistol to defend themselves as they escape (i.e., no duty to act).