In the March edition of the Sleep Journal, there is an article about the effect of sleep restriction on cognitive performance of children with ADHD. In this study, researchers studied children with and without ADHD. They measured the kids' performance on cognitive testing while getting normal sleep at home and after getting about an hour (average 40.7 minutes) less sleep per night for six straight nights.
The results showed that kids with and without ADHD experienced deterioration of performance on cognitive testing with sleep deprivation. The researchers had the children with ADHD stop their medications 48 hours before the study. It would have been interesting to see if the kids with ADHD were able to improve their cognitive performance on their medications despite sleep restriction.
I think the point of this study is that sleep deprivation could worsen academic performance if kids are sleep-deprived. In our society, many pre-teens are sleep deprived, at least according to the amount that sleep experts say these kids should be getting. If you have a child with ADHD, make sure they get plenty of rest.