Why do teachers consistently reject the credibility of Wikipedia?

I don't think Wikipedia is not credible.  I direct my high school students to it to get an overview of what they are researching, and then tell them that (for a short paper or presentation) I want citations for at least three more sources.  And then I check to make sure that they really used them.

If you are doing research, then any encyclopedia would only be good for base definitions (and then, you might as well use a dictionary).  University writing is meant to be academic, and is meant to promote finding the sources yourself and coming to a conclusion.  Wikipedia already does this from a certain perspective.  It takes all the information about a subject and attempts to create an entry based upon evidence to support it.  It's not so much that it isn't credible as that it doesn't make you credible by using it.  Your arguments as the author of this question are quite correct.  Wikipedia does have abuse standards (as I have found out through trolling Wikipedia at some point as a teenager).  However, Wikipedia is meant to be an interesting source of information collected for general education.  It isn't meant to be a place for you to quote to support arguments.  Any academic research that is done purely through encyclopedia entries will most likely come out very flawed.  If you have trouble finding sources though, and find some information on Wikipedia, then go to the sources at the bottom of the page and look at them yourself.

Wikipedia has a poor reputation in the K12 schools I have worked for and is considered by many an unreliable, possibly misleading source.  I don't do much research in my courses, but I know of cases where teachers do not allow students to cite Wikipedia as a source. I can't tell you precisely how this got started or widespread this idea is.  I'm also not justifying it.  I like Wikipedia and I know it's a site where you can find helpful general overviews of broad topics and specific bits of information that may otherwise be hard to find.  However, I agree that it shouldn't be used for research assignments. In my opinion, a main reason for this issue is the dynamic nature of the content.  Sometimes articles are flagged as incomplete, not sufficiently well-documented, biased, controversial, etc, as a consequence of the crowd sourcing of the material, some instances of abusive editing, etc.  But I think the basic reasons have more to do with the learners and politics. First, middle and high school students usually are not knowledgeable enough, mature enough, or conscientious enough to judge the quality of a source as effectively as they should.  There are students who will find absurd nonsense online and call it research because they either don't know any better or (more likely) just want to minimize effort. Second, if a student writes absurd offensive nonsense about a sensitive topic (say, slavery or the Holocaust) the school and district may wind up on the national news and be forced to drink the hemlock even if, objectively, it's not their fault what an individual student wrote on a piece of paper for a history assignment. In one school where I taught, the principal personally removed some library books from the library because they contained historical photos of slaves.  The photos were all portraits-- there was nothing about them per se that was problematic.  The story was that a student had innocently used photocopies of some of the photos in a history project for a history fair and a second party found this use objectionable.  Yes, OK, censoring the library is fundamentally wrong, but administrators believe in CYA because the media is eager to pillory them for just about anything, whether it's true or not, within their control or not, required by regulations or not.  I also know of cases where teachers provide the students all the sources to be used simply to avoid any controversies about the material and plagiarism.  (Yes, I'm serious) Keeping yourself out of the newspaper is a very high priority, and if directing students to a source other than Wikipedia might help, the schools are happy to try it.  They are controversy averse because the public forces them to be.

I answer the same question in It answers how [quote on quote] "reliability" is consistently used to defend reputation and keep publishing profitable, as well as filters out newcomers, amateurs,  academic rivals, and open access. It also shows how credibility is still derived from "respected" authority rather than proof via research.