My high school start time was 7:15 and while I am and have always been a very good driver, a great student and also went to bed on time, I had two accidents in HS due to sleep deprivation and had to get my mom to drive me to my college class (joint enrollment student, 20-minute drive every afternoon) because I was so tired I couldn’t drive in the afternoons anymore.

Even 30 more minutes of sleep (in the morning) makes a huge difference for these kids.

So they get sleepy later, right? but when do they go to sleep? after they’ve beaten the latest video game, texted all their friends, watched Stewart AND Colbert?

There is a sufficient body of evidence showing that adolescents experience changes in social context and physiology at the onset of puberty that cause them to develop an evening preference chronotype, predisposing them to go to sleep later at night and wake up later in the morning than children and adults (Crowley et al., 2007); this evening preference chronotype is not congruent with early school start times. The asynchrony between school start times and adolescent circadian rhythms can lead to an accu- mulation of sleep debt (Carskadon, 1999, 2002), is associated with cognitive impairments, and appears to adversely affect behavior, subjective levels of sleepiness, subjective levels of depression, and school-related variables such as attendance and tardiness (Dahl, 1999; Dinges & Kribbs, 1991; Wahlstrom, 2002). In accordance with this body of research, some schools have delayed school start times, with promising results. Adolescents who attend schools with later start times have been found to sleep more per night, resulting in a more positive tem- perament, fewer behavioral problems, and an increase in corre- lates of good academic performance (e.g., attendance, continuous enrollment; Center for Applied Research and Educational Improvement, 1998a; Owens et al., 2010; Wahlstrom, 2002).

Instituting a delay in school start times may be accompanied by complications related to transportation costs in some school districts, although this is a much lesser issue for high schools that already require students to use alternative transportation meth- ods (e.g., public transportation, car pools). In addition, changes in start times are associated with disruptions of extracurricular activities and other social and leisure activities in many school districts. However, with sufficient strategizing and preparation, the inconvenient consequences of changing school start times can be attenuated. There is evidence that with adequate planning and preparation, school boards have been able to delay school start times at acceptable monetary cost (given the enormous potential payoff) and tolerable disruption of community functioning.

Here is the citation:

School Start Times and the Sleep−Wake Cycle of Adolescents : A Review and Critical Evaluation of Available Evidence Matthew Kirby, Stefania Maggi and Amedeo D’Angiulli
EDUCATIONAL RESEARCHER 2011 40: 56

The other problem is that not every parent is going to make their 15 year olds go to bed at 9:00. I would guess the vast majority won’t. CSD is accountable to the state and feds for their achievement, or lack thereof, regardless of whether parents are willing or able to impose an early bedtime. My guess is that the kids most at risk will be the ones most affected by this.