It would seem that a few hotbeds of intellectual progress that are public schools have decided to restrict wikipedia above and beyond the general criteria of "don't cite *cough*plagiarize*cough* Wikipedia for any research assignment."
Me, I generally like Wikipedia, but I view it as a starting-off point. Sure, there's plenty of subjects where I just need to get the gist of something, and for that it's pretty darn swell (even have the search form on the main page on my internal webserver). Yes, I agree that the crowdsourcing effect does not guarantee accuracy by any stretch of the imagination, and often can give mistruths due to it being "popular" rather than "correct"
But more so, I think it's wrong-headed to actually block it from school computers. Seriously, what if you wanted to do a report on such fine literary classics as Doctor Who or Batman? What then?