A relevant excerpt from the article for you: "Children ages 10 to 12 need a little over 9 hours of shuteye a night. But it's up to parents to judge the exact amount of rest their children need and see that they're in bed in time for sufficient sleep."
One of the questions I used to ask teachers at back-to-school night or open-house was "what are the three most important things we need to do as parents to make sure our kids are good students?" The #1 or #2 answer was always "Make sure they have a good night's sleep." So I stopped asking. The answers were all the same.
My 12-year-old daughter gets about ten hours a night. Any less and her mood is darker then next day.
Oddly, or maybe not, she has less trouble getting up on weekends than on school days, even though she gets less sleep on weekend nights. She says it's because she has things to do when she gets up on weekends. I wish I could find that kind of motivation for her on school days as well.
The child will need at least 8 hours of sleep. However, I always ensure that my daughter is in bed half an hour before so that she is restful and peaceful which then allows her to have a good night's rest. She wakes up refreshed and in a happy mood.