I believe that a big part of the reason that high school teachers don't allow it is that using wikipedia does not add value in your education. One of the most important things that I learned in High school (before the internet is what it is) is HOW TO FIND THINGS. It is pretty easy now, but if you were allowed to use that as your only source, you would never know how to go look in books, peer reviewed journals, articles, etc. You have to think that the teacher's job is the educate you...not just on the facts, but how to find them. Four years of education in an engineering program and I remember or have used maybe 10% of the FACTS....but the simple process of how to learn, how to think, and how to FIND the answer is what I really got out of the experience.
Think of it this way....how accuracte is a textbook really? It is written by one organization, with a staff of experts. Those experts don't change from year to year. If they aren't up to snuff in their researching, how accurate is the book? There are no real checks and balances from the industry or the REAL experts...not just the person who wrote the book (I refer PRIMARILY to high school books...which are of a MUCH different nature than College and graduate books, which are most often WRITTEN by the experts). I have found MANY MANY errors in text books. The difference is....I can't fix it there. I can't even provide the necessary feedback - and if I did...who would care.
The beauty of Wikipedia is that if you DO find an error, it is your right to go and fix it. Providing a constant, ever evolving system of checks and balances. Especially with a subject like history, which is ALWAYS changing and evolving. History is not necessarily written in stone. It changes more often than you'd think.
Lastly....what year was your textbook written in? When I went to school, they were all AT LEAST 5 years old. The Average wikipedia entry CAN'T be more than 5 years old.