The History of the World books and study guides are recommended for 9-12 grade and adult learners. So, they would be perfect for your 9th grader. We recommend that you use The Story of the World with your 7th grader along with your younger students. Follow the recommendations in the front of each activity book under "Multilevel Teaching." This will tell you how to make the material more challenging for a middle school student.
Including your 9th grader in the SOTW probably won't be helpful for him/her. Although the time periods are the same (Ancients, Middle Ages, etc.), the older books cover much more information and don't really line up or coordinate with the SOTW books. However, extracurricular activities like going to a museum, watching documentaries will line up if you place them all in the same time period.