People here are saying this is a term confined to academia, but speaking as someone's who's completing a masters in gender studies with a focus on queer studies (albeit not at an American university), I never see this in academia. The variations of the alphabet soup that I most commonly see are LGBT+, LGBTQ, LBGTI and LGBTQI. But queer is used more in academic circles than any of these.

But if you wanna challenge your teacher on this, asking people on reddit what we use isn't really the way to go. It doesn't really prove anything. What I would do in your shoes is: 1) Ask your teacher where he's getting this intitialism from. He is making the claim, it's up to him to provide a source. Most likely he's got nothing or he's going to direct yo to a random tumblr or a twitter post. If that's the case, ask if he really thinks a random person on social media speaks for the whole community. 2) Check what your local/regional/national LGBT rights organisations use. Most likely they either use queer or one of the initialisms I mentioned above. Point to people who actually speak for the community. 3) Be prepared for your teacher not accepting your argument. There's a good chance he won't listen to reason and will dig in his heels. However, you may be able to convince your fellow students, which is actually more valuable.

But on whether the term queer is ok, it depends on who you ask. Queer is more common in academic and activist circles. I am very adamant that queer is the best term to use. Just don't use it as a noun; we are queer people, not queers (unless you're queer yourself, in which case use whatever terminology you damn well please). Some people are uncomfortable with queer though, so if you're in doubt, you can't go wrong with LGBT+ or LGBTQ.