Preface: I'm not in favor of book ratings.
Tony Buchsbaum at January Magazine has this post called "Are Your Kids' Books Rated R?" In reading this article (and others like it), I'm surprised that no one ever seems to mention that manga has ratings ("all ages," "teen," "older teen," and "adult"). How does this play into arguments about marketing, sales, censorship, artistic expression, selection, etc.? I have to say that I personally really like the ratings because I don't read manga myself and there aren't a lot of reviews for manga in the major library and book review publications. I rely on these ratings to give me a sense of what age group the book is for. I have all but "adult" series in my library. I could be wrong, but I think it might actually help in a book challenge because I could say that students chose a book that was clearly intended for above their age, or we do have books for older students who go to my school. I don't know---maybe that wouldn't help.
I do agree with him that it is often difficult for students to select books that are of appropriate age, style, interest, etc. to them (many things to consider as far as "appropriate," not just if there is anything naughty in the book). I posted about the research Vivian Howard did for VOYA on how selection is the single biggest impediment teens face in reading books. I remember seeing a 9-10 year-old boy reading Frank Portman's King Dork. I can totally see why he'd select that book, but it's totally not right for someone of that age for many reasons. Then, of course, we have the perennial issue of parents who purposefully choose books intended for older individuals because their child has a high reading level, even though the "intended audience" isn't merely about vocabulary and sentence structure. Don't really know where I'm going with all of this except to bring it back to my original question/observation of the fact that manga has ratings and is that bad?