This is a tough call. The teacher probably put those books on his desk as a way to start a reasonable discussion on the topic. Well, okay.
But if this teacher is allowed to keep those books on his desk, could a Muslim teacher keep the Koran on his desk? You know, for discussion purposes (aka, conversion purposes).
I would support having those books in the school library. Libraries should have books with lots of different points of view.
Having them on his desk (presumably in a classroom or his office if he has one) is harder to support. IMO those places should be neutral, so that any student could feel comfortable (or at least not challenged). Would I as a student feel comfortable if the teacher had a gay pride flag in his classroom or in his office? No. So keep those places neutral -- reasonable discussion can still take place.
BTW, whatever happened to reading, writing, and 'rithmetic?
If they allow other teachers to keep books on their desk that are in support of LGBT stuff (Billy has 2 mommies or whatever), then they should allow this teacher to have his books on his desk. Otherwise, this is viewpoint discrimination.
Nothing tough about it. They rot will continue until it is stopped