Several students (in this case they were from Leadership) are chosen to be victims. A crash scene is constructed on the campus, much more realistic than what you see on ER or Third Watch because the people who do this are cops, firefighters, paramedics and EMTs. The 'dead' are taken away by the coroner. The 'injured' are taken by ambulance or helicopter to the closest hospital; these students don't go home that night and don't go to school the next day. Every 15 minutes during the school day, a 'grim reaper' removes a student from class. There is a memorial service and obituaries.
This website isn't from my school, but it has some very good pictures and material.
Some people will probably say that it's too much, too harsh on the kids, but make sure you check out the guestbook section of the site. It really does make an impression, and if even one life is saved as a result, I think the whole production is worth it.