It is not a book that is trying to convince the world that transgendered people are everywhere and you should be one too. It is not trying to convince its audience to start cross-dressing. It is not trying to push for greater acceptance of transgendered people. This is not a book with an axe to grind. And even though the title character, Luna, is a person who was born a boy (named Liam) but is convinced he wants to be a girl, this book really is not about him. In fact, if it were up to me, I would have named the book Luna's Sister.
Luna is the story of Regan, a girl whose life has been turned upside down by her brother telling her that the driving desire in his life is to be a girl. As far as Regan knows, she is the only one in her family and in the community who Liam/Luna has told. She is also the one who helped him after a suicide attempt. And so, Regan's love and concern for her brother has become a full time job. She helps him when their dad insists he try out for sports. She helps him when he decides he want to try passing for a girl at the mall. She helps him when he makes contact with a transgendered person on the internet and wants to meet with her when she is in town for a conference.
What ends up taking a back seat in Regan's life are her own wants and interests. She loves her regular babysitting job, but eventually loses it because she has to cover for her brother. And when, in science class, she meets a boy who actually seems to like her for who she is, she has to keep putting off going out with him because she has to help her brother.
In the end, I would argue, this is about the universal struggle that any high-school- aged kid goes through when someone they care about is dealing with something that is all-consuming and that thing is breaking apart their family.. It would be a great book for a kid with struggles like that to read and feel they are not alone.
But the problem is, this is a book with a transgendered character, and realistically, it is bound to be challenged if you use it in your classroom. It is bound to be challenged if you put it in your school library (unless, you have a policy defending a range of perspectives). In fact, it might get you into trouble if you keep it in your classroom library. When I was teaching high school, this would have been the sort of book I might keep behind my desk and loan out to someone who I knew could handle it.
Maybe that seems cowardly, and maybe it is. But I think you need to ask yourself how you can best help your students who struggle. Then you need to ask yourself if you can still be that helpful if you lose your job. Sigh.