IMO, a proper school shooting game would probably be somewhat of a life simulator that ends with an action minigame.
Imagine that you play as a character that has, like, five months until school ends (150 days). During that time, as you play as the guy, you can assemble weapons (like homemade daggers, pipebombs and stuff), make money (and lose it when you're mugged by bullies or something unless it's hidden away), buy weapons and supplies, and try to cover up the stuff away from authorities until the action. Perhaps it would also involve making allies who would fight on your side on the shooting day. The shooting itself can be initiated on any day as soon as the player has any weapon at all.
It would probably have the peaceful ending, where what matters is how many friends and money was made.
But for obvious reasons, that would probably be somewhat of an alternative ending that would require no guns or anything. The real ending is the Action part, which can be turn-based kinda like in "Mission in Space" (http://www.newgrounds.com/portal/view/588939). The enemies would consist of Kids, Teens, Adults (unarmed), Police, SWAT and, perhaps, the military. The Action part is endless with the enemy set getting stronger, and it would continue until the player and all allies die.
At least that what comes first into my mind when I think what would be the perfect school shooting simulator with all the preparations and stuff...