I have to say that I don't completely trust these various rating systems. I find it odd that the Percy Jackson books are consider 4th grade while the Wimpy Kid books are 5th grade, especially when you compare them to other fifth grade books like Chronicle of Narnia and the early Harry Potter books.

Half of the second graders in DD/DS class last year were addicted to the Wimpy Kid books and reading them early in the school year.