If you are talking about a person firing a gun on stage, I would recommend a good quality blank gun. You can get these from theatrical supply companies are are fairly in-expensive (under $50), reliable and relativly safe. I use a .22 caliber one in our 150 seat theater and it makes a pretty good sound, not to loud, not to quite.

Even though they are safe, like any weapon used on stage, they must be treated with care, and everyone involved in the show must understand it's not a toy, especially when dealing with younger actors.

Dan

A starter pistol or a real gun with shells loaded with only the igniter are always more authentic and startling, but since it is to be used in a SCHOOL and the "crazies" have made it neccessary to treat EVERY weapon (even stage props) as a threat to school safety, THAT will probably be met with resistance. As far as the SOUND of a gunshot goes, you can replicate that with a "slapper board" which is nothing more than two clear, flat, smooth boards, freely hinged on one end with handles on opposing outside surfaces. To make the sound of a gunshot you merely slam the two board surfaces together rapidly. Different lengths and widths of the boards will give you different pitch and timbre, so experiment with the "perfect size." As far as the stage prop gun, many plastic, "toy" ones have enough detail and just need to be painted for believability. Then the only trick to getting the audience to believe it is to coordinate the gun actions with the "slapper board" off-stage.