Here is an article about sites such as CSM: http://io9.com/the-thriving-industry-that-helps-encourage-book-censors-1641312927. Read the comments as well (lately I have been reading the comments first and then the articles) as they contain some real gems: "I really don't have a problem with censorship in schools. Children are not adults, they shouldn't have access to anything and everything that was ever written."
But the entire article is an interesting take on why Common Sense Media and other similar sites tend to thrive: they fill a perceived need. Parents too busy to read or check out a book for their little darlings? Rely on someone else to tell you. Better yet, go to an anonymous source where the way rankings are determined by "experts" who are not identified. Here is the description of how ratings are concocted: https://www.commonsensemedia.org/about-us/our-mission/about-our-ratings.
Take a look at the page all about SPEAK: https://www.commonsensemedia.org/book-reviews/speak. Notice that one alternative to SPEAK is THE LOVELY BONES, please. Note that under What Parents Need to Know are these little criteria with circles indicating violence, drinking, sex, language, etc. This reminds me of the scene in DEAD POET'S SOCIETY where Robin Williams mocks the x and y axis ratings of poetry. That is what is happening when we break down books into component pieces like this.
THE CHOCOLATE WAR is referred to as Dangerous Liaisons for teens. Really? THE CROSSOVER has the reading age as 9. Really? And let's not even talk about the missing titles as there are plenty.
In short, if you want to know about a book and its appropriateness for your kid, ask someone like a librarian or a teacher. Check out the reviews. Chances are, though, if your kid is asking for the book, there is a reason. Talk to him or her and find out why. And remember, the MPAA ratings have more to do with language than sexuality and nudity and violence. How could we expect a system to do well if we consider things out of context?