Some teachers discourage the use of Wikipedia by their students, but usually the grounds for this are quite specious and show a complete lack of understanding by the teachers of what Wikipedia is for, and how useful it can be as a learning tool.
Wikipedia is the most up to date encyclopedia available in the world today. There are events happening around the world right now which will be updated on Wikipedia within seconds, and not appear in print encyclopedias for months or even years. Yes, it is true that anyone can create or edit an entry in Wikipedia, but the research shows that the vast majority of the people who do these things actually do know what they are talking about, and there are excellent checks and balances from an army of people around the world, all volunteers, who monitor changes to Wikipedia and can quicky restore or delete changes to the site which are misleading or malicious.
Teachers should teach their students how to use Wikipedia, not ban it. Wikipedia itself has a very useful page which sets out how the site should be used by students and academics, and a visit to this site would be an useful part of every ‘introduction to research in this topic’ lesson. The Wikipedia pages about this are here:
Wikipedia is a tool for research, and a very useful one. That is why it has become the first place that many students in all levels of education, and I suspect many teachers as well, go to find the basic information and key web resources about a topic. I tell my students to use Wikipedia, but to use it properly. It is not a primary source, and so it should not normally be quoted in a reference list/bibiliography. But it is one of the most useful ways available for finding the references which should appear in a bibliography, and for students familiarising themselves with the key concepts and vocabulary of just about any given topic.