Julia F. Dewald, Anne M. Meijer, Frans J. Oort, Gerard A. Kerkhof, Susan M. Bögels The influence of sleep quality, sleep duration and sleepiness on school performance in children and adolescents: A meta-analytic review Sleep Medicine Reviews, Volume 14, Issue 3, June 2010, Pages 179–189 http://dx.doi.org/10.1016/j.smrv.2009.10.004
This paper is a meta-analysis of the relation between sleep and school performance. Meta-analysis is a statistical method combining different study results. It enables the discovery of consistencies in a set of seemingly inconsistent findings.
Sleeping well is important for children and adolescents, since it influences learning and other memory processes.
insufficient or low quality sleep during (early) adolescence impairs the executive function of the prefrontal cortex16 and consequently the decline of learning abilities and school performance.  and [*18]
Sleep is crucial for children and adolescents’ learning, memory processes and school performance. [*1],[*2] and [*3], but 45% of children are not getting enough  and . On average, adolescents need 9 hours of sleep per night. But as sleep quality is more important than sleep duration, more effective to measure is sleepiness, as this relates directly to enough sleep or not. If a child would sleep 8 hours a night and would not be sleepy during the day, their school performance would not suffer, in contrast to someone who could sleep 9 hours and still be sleepy in school.
Insufficient sleep might be caused by an interaction of intrinsic (e.g., puberty, circadian or homeostatic changes) and extrinsic factors (e.g., early school start times, social pressure, academic workload) leading to later bedtimes while getting up times remain unchanged. Additionally, it is known that approximately 20–50% of children and adolescents report daytime sleepiness.  and 
We see that daytime sleepiness is highest among children in mid-puberty. This makes me wonder what age group mid-puberty is exactly, so more about that later. The ages of mid-puberty also differ between boys and girls.
mid-pubertal adolescents may need more sleep than younger or older adolescents in order to reach the same level of daytime alertness and neurocognitive functioning. [*2], [*16] and 
The article ends with a short summary, which is nice to remember where to focus:
Poor sleep quality, insufficient sleep and sleepiness are significantly associated with worse school performance.
We recommend educating children, adolescents, parents and schools about the importance of sleep for school performance. As part of this, education about sleep hygiene can be given in order to improves the sleep of children and adolescent and consequently school performance.
Attention should be drawn to the development of prevention and treatment programs that focus on the sleep of children and adolescents
For more literature on sleep, see this post.