I ride in western Massachusetts, and I have had college students yell things like that at me. It has nothing to do with whether I, or other cyclists, am law-abiding; it's about college students being jerks and thinking it's funny to harass people who look different, and because now that they can drive, they think bikes are for kids.

I suspect it's the same with the high school students you mention. If they say that to you in school, though, where you can respond to them, you've got a great opportunity to teach them otherwise: if it's illegal to ride on the sidewalk in your town, tell them that. If it's legal, explain why it's usually a bad idea (pedestrians feel threatened by bikes going 15-20 mph, drivers don't expect fast-moving vehicles to be crossing driveway cutouts, sidewalks are often in poor shape, etc.). Point out what the Vermont Driver License Manual says: "Under state law bicyclists have the right to operate on all roads, unless they are specially prohibited, such as on limited-access highways like the Interstate.... It is important to remember that bicyclists have the same rights and duties as motor vehicle operators" (p. 40), and in the instructions for bicyclists, "Always ride with through traffic on the right side of the road, not on the sidewalk" (p. 41). Tell them that they may well be tested on this when they take their driving license exam!

In short, treat it as a teachable moment.