About the
stigma: the "short bus" can be a back-handed way of saying that someone isn't that bright, e.g. "Yeah, well I'll tell ya something about Bill, I mean, I'm not saying he isn't a great guy, but he definitely rode the short bus to school. If you know what I mean (
nudge nudge wink wink)."
Short buses are often used by private schools and preschools that don't have to transport as many students as larger public schools do. Churches and construction companies are also fond of surplus short buses (and even full-size cheese buses for that matter), because they are cheap and can hold many people.