In addition to Greg's answer, and to be a bit more blunt and specific, Baldwin's book is frequently still banned because of its frank discussion of homosexuality, a taboo in many high school curriculums. For example, 14 year old John masturbates in a bathroom while thinking of other boys. Though he is ashamed of his emerging sexual orientation, (he says he "watched in himself a transformation of which he could never speak") his shame is not enough to quell his desires. Even his Sunday school teacher is not immune from his fantasy life.
There are any number of reasons why a book might be banned, and this might vary from region to region. This book has been banned or challenged for at least two distinct but related reasons.
In New York, it was challenged because of rape, violence, and how women were treated. In Virginia, it was challenged because of sex and profanity.
See the website below for a few more details: