Is Wikipedia an acceptable research source for elementary and middle school students?

My school has a policy that students may not use Wikipedia as a source for any school-related research. I was part of developing this policy, based on the fact that anyone can contribute to Wikipedia without credentials. Well-publicized incidents of people posing as experts on Wikipedia have been the impetus for this policy. We are trying to teach students what makes a source reliable and how to choose reliable online sources even when you aren’t familiar with the author or website.

I have, however, read about schools that allow Wikipedia as a starting point for research. For example, I see that the article on Wikipedia mentions X, so I can search for that on more reputable and reliable sites. Wikipedia would not be the final cited source for that information, but it gave the student an idea of what to look for. I’ve also read about schools that have done studies of the accuracy of Wikipedia articles, and have apparently found that Wikipedia is pretty good about fact-checking.

I’m curious to know what other schools do to address this question, as well as any problems or success you have had with Wikipedia in the classroom. (I use Wikipedia all the time myself, outside of school.)