Many parents have told their kids that they are to leave the school building if there is any lockdown announced, on the theory that it is harder to hit a moving target than a quivering one. Some younger kids most likely will not do so, but I'm willing to bet that those in high school will be more than happy, willing, and able to do so. Let the school and parents fight over suspension/expulsion issues afterwards - always keeping in mind that the kid is alive to be suspended/expelled.
Sometime after one of the fist contemporary school shootings there was some talk of training kids to throw books at any shooter that entered the classroom, to dogpile them, and to beat them with backpacks. A few schools tried out this plan under some sort of grant funded research and amazingly found that volunteer active shooters who wanted to wipe out the classroom/building had a hard time doing so under that sort of pressure. Sadly, "if it saves one child's life" seems to have won out over "doing this may improve the odds that some kids will survive".
Correct me if I'm wrong, but it seems that of late "active shooters" have committed suicide well before the cops arrive. I guess the choices are to hope that they only kill/wound a certain percentage of the school and them off themselves or to have at least one person in the school who can and will run to the sound of the gun and increase the desire of the active shooter to become dead.
I know, saying that is not allowed. So sue me.