Here is the American Library Association's webpage for it. Here is their page on the most frequently challenged books and authors for 2007. Toni Morrison's books come out of the top 10, but she still remains the fourth most frequently challenged author. 

As far as the legality of this stuff goes ... it's legal to ban books if that is what the school board wants to do. The only exceptions to this are that if the school board has a procedure in place that it does not follow (see this case) or if the board is discriminatory against a particular group or viewpoint (see this case for instance) in their decisions. But, most of the time, it is perfectly legal to ban books from schools. 

(I got some kind of timing by the way; I am teaching instructional issues this week in my school law course. Hope my students are paying attention!)