I can't figure out why I can't find this, but...
Protagonist is high school student, I think in New York. Either his county or the state has been "locked down" by Earth First. No pickups and CAP-eligible folks can't leave. He secretly organizes resistance, including killing Earth Firsters. High school becomes a pre-pack sorting scene with rooms set aside for test drives. Arranges shooting lessons for his friends. At the end, the authorities led by the Governor (maybe a Senator?) raid the school and have him and everyone else in the gym and he pulls a hidden gun and shoots the Governor.