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I ask this question because in school, students are always told "do not quote wikipedia" or that "Do not believe everything you see on Wikipedia, we are teachers and we know when something is wrong there". (The latter being more common in my case) Anyway, Wikipedia does have an article about it here. Still the question stands about whether it is reliable, and if it is, when or if it will be permissible in the classroom.
Have any studies been made on the reliability of Wikipedia?
In December 2005, the science magazine Nature conducted a study to determine whether Wikipedia was as accurate as traditional encyclopedias, namely the Encyclopedia Britannica.
In Study: Wikipedia as accurate as Britannica, Daniel Terdiman summarizes the study results:
Wikipedia is about as good a source of accurate information as Britannica, the venerable standard-bearer of facts about the world around us, according to a study published this week in the journal Nature. ...
For its study, Nature chose articles from both sites in a wide range of topics and sent them to what it called "relevant" field experts for peer review. The experts then compared the competing articles--one from each site on a given topic--side by side, but were not told which article came from which site. Nature got back 42 usable reviews from its field of experts.
In the end, the journal found just eight serious errors, such as general misunderstandings of vital concepts, in the articles. Of those, four came from each site. They did, however, discover a series of factual errors, omissions or misleading statements. All told, Wikipedia had 162 such problems, while Britannica had 123. That averages out to 2.92 mistakes per article for Britannica and 3.86 for Wikipedia.
Of course, what makes Wikipedia different from a standard encyclopedia is that it can be updated by anyone and those updates appear immediately. When there's a particular hot topic item in the news, it's not uncommon for its Wikipedia entry to be updated rapidly, both by level-headed and factual participants and by more extreme, agenda-driven actors on both sides of the issue at hand. Consequently, when you read an entry at Wikipedia it's important to bear in mind that what you are reading now might include some dogmatic if not outright incorrect statements.
As Bibhas noted, it's important to check the sources and references. I also would encourage you to note the history of the piece you are reading. If there were many edits in a short window of time that may be a sign that there is some back and forth going between two "sides," which could imply that the information presented might not be as objective as it was prior to the volley of edits.
The source of the data in Wikipedia is people like us, who knows about a certain topic or domain. And the sources of those data are shared at the bottom of most article. (For example, http://en.wikipedia.org/wiki/Horse#Sources.) You can check that on every single page as it has a discussion section where questions against the data, if any, are raised and proofs are provided if necessary (http://en.wikipedia.org/wiki/Talk:Pearl_Harbor). You yourself can raise question if you want to, they'll answer.
Hence, quoting Wikipedia is far more reliable than quoting a teacher him/herself. As the data is available there in each and every article, it has been filtered for years by millions of teachers like the one who teaches you.
If your teacher still don't agree, go to Wikipedia, check the sources of their data at the bottom of the page and provide that to the teacher.
P.S. - If sources are not available, check the references. They are always there...
I have personally edited incorrect information on Wikipedia. Corporations are known to use Wikipedia as a tool to steer their public image. Sinbad is still among us. Still, wikipedia is great as a preliminary survey on most topics. I use it all the time. Just don't use it as a source on an academic paper and you'll be fine. Your teachers and/or profs are trying to get you to dig a littler deeper, that's all.
Wikipedia is an excellent source of general information when starting research. An an example, I'm writing a review of a paper about Leishmania, and the Wikipedia articles that are related and relevant to me (Leishmania, Vaults, 7SL RNA) are quick and easy reads, and loaded with great references to further papers which makes finding them straightforward if not trivial. I might even use the same fact as the Wikipedia article, but it's senseless to cite WP, which in-turn cites the origin of the fact.
Outside of the more scientific topics, however, the amount of references can steeply drop off, which means it won't be of much direct help for any formal research, though it can still help familiarize you with the topics.
Wikipedia articles are not uniformly edited by any stretch of the imagination. Making generalizations about Wikipedia as a whole based on some sort of average quality metric of their articles is dubious.
If anyone (e.g. your teachers) say that "Wikipedia is wrong on point X, as it's Y", that doesn't absolve them of the need to provide some sort of proof or evidence on it as well. Sure, you might be inclined to believe some source above another, but belief shouldn't mix with fact.
Here's Wikipedia on The reliability of Wikipedia:
A notable early study in the journal Nature suggested that in 2005, Wikipedia scientific articles came close to the level of accuracy in Encyclopædia Britannica and had a similar rate of "serious errors".[5] This study was disputed by Encyclopædia Britannica.[6]
In areas where I actually have expertise, Wikipedia does quite well. They're not perfect, and you have to watch out for articles that are in the midst of 'editing wars', but if you want to know about RuBisCO or Gamma-ray bursts, Wikipedia does a bangup job.
