Chance of getting caught carrying a gun to school, well, it depends I suppose on how it is carried. It also depends on how stringent the school is in routine checks. If rapid deployment of the firearm is not the highest priority, then it can be fairly well transported in secret on your person, or in your school bags. I don't remember ever being searched on any campus, whether I was armed or not, so unless you are foolish and make it's presence known to others, or the school institutes regular and vigorous searches of personal effects, then I'd say the chances of being caught carrying a firearm to school are pretty low, and reasonable enough to warrant the risk.

What other choice do you have? To not carry is to remain a defenseless victim, no matter how minimal you think the threat may be. As was proven here, you can't rely on security to keep you safe, nor the police; even the justice system failed in this case. Unless you are willing to put a cop in every classroom and hallway, which is even less practical, there are no other options left.