I've spent the last hour looking at this some more. I've looked at schools in these categories http://en.wikipedia.org/wiki/Category:High_schools_in_Fairfax_County http://en.wikipedia.org/wiki/Category:Public_schools_in_Montgomery_County%2C... http://en.wikipedia.org/wiki/User:Aude/schools - Results show most other schools in these systems are much worse offenders and have experienced long-term blocks (3, 6 months or so). One was even indefinite, which I'm not sure is okay?
There appears to be one proxy server per school, thus one IP address per high school in these jurisdictions. So by "school", I mean each high school. I should note that not all school systems are set-up this way. In contrast, Arlington County, VA doesn't do that. They have multiple IP addresses for each school. I'm not sure about others. Prince George's County (MD) also has multiple IP addresses per school, though each there has been blocked ~3-5 times. For some reason I can't figure out, I haven't come across vandalism from/to public schools in Washington D.C. (even though I also watchlist them)
Anyway, a long-term block might be in order for the specific school in question. This essentially does the "Sorry, your IP comes from a network know for anonymous vandals..." and we have {{schoolblock}} template.
Though, the vandalism does appear to come from just one or two students. If there was another way of dealing with the problem, such as restricting specific students from accessing Wikipedia (if that is any good?) from school. Don't think it can 100% be stopped, but would vandalism from the school be significantly curtailed? I think it would be interesting to find out if the school can do this and if it would help. If these kids were blocked from Wikipedia (including for doing homework, and all) would this be enough to convince them to stop? or maybe these kids don't care about homework? Or this might be too optimistic.
-Aude