Your link had nothing to do with LGBTQ characters, "predominantly" is certainly not the same as "universally", and books meant for high school student may contain topics not appropriate for younger children.
Think of how George shoots Lennie in "Of Mice and Men", or the racism in "To Kill a Mockingbird", or why Hester Prynne must wear a scarlet "A", or when Anne Frank talks about her menstruation. Or the details of the meatpacking industry described in "The Jungle".
They are all taught to high school-aged kids. I don't think most people would want all those details described to elementary school kids.
I can't figured out why your assertion is relevant.
While it may be true, surely you don't think that only white hetereosexuals can be portrayed as something other than milquetoast-bland in high school literature.
