Was it a gang of teenage girls? Cause that sounds like The Violent Years. If it wasn't then I don't know. Could be Teenage Crimewave, as it is one of a handful of MST3K's I've never seen. I also think that Teenage Strangler's climax takes place in a school, but I don't remember if there was a shoot out.
And I love The Giant Spider Invasion too. "Wooo PACKERS!!!"
Also if anyone cares, and I know none of you do, you can buy MST3K Vol. 23 at your local Wal-Marts right now. It is in the TV section. Other then the movie, I think this is first time Wal-Mart has carried the show in their actual stores and it is a trend I'd like to see continue since I have to either order online or plan a trip out of town to get them usually.