Pediatric Neurology recently published a study on children who grind their teeth while they sleep. Teeth grinding, or bruxism, is linked to a variety of disorders but is commonly thought of as an outlet for stress. Research written by Vendrame et al conclude that children diagnosed with migraine headaches also suffer from sleep disturbances. In my opinion, though, this wasn't the most important finding in their paper. In the course of discussing this phenomena, the authors highlight an important issue: Children suffer from tension headaches.
Often this is an ignored arena for parents. A lot of adults in this "shake it off" society seem to think that just because kids are small, (or don't have jobs, financial concerns, etc.) they don't have stress. Hey, between school shootings, instantaneous gossip via IM/text, the public humiliation readily available on Youtube (the #1 search result for "embarrasing" probably hadn't gone to her first prom yet), and parental pressure to perform academically/athletically/socially, I think there's plenty out there for a kid to get stressed out about. In addition to stress, tension headaches have been associated with risk factors such as high caffeine intake and anxiety, all of which can be found running rampant through the halls of most middle schools and high schools (Ohayon 2001). It shouldn’t be surprising then that the bodies of half of those children experiencing tension headaches are finding the only outlet for overactivity they can manage- their engines are still revving when they're supposed to be sleeping, and hence they're grinding their teeth at night.
References:
Ohayon, MM., Li, KK., Guilleminault, C. (2001). Risk Factors for Sleep Bruxism in the General Population. Chest. (119) pp 53-61.

Vendrame, M., Kaleyias, J., Valencia, I., Legido, A., Kothare, S.V. (2008). "Polysomnographic Findings in Children With Headaches." Pediatric Neurology. 39 (1) pp. 6-11.