A very useful infographic on the dangers of sitting, here:
http://www.mindbodygreen.com/0-2420/Sitting-Is-Killing-You-Infographic.html
and a BBC piece on the same topic:
http://www.bbc.com/news/health-32069698
Vox.com chimes in:
http://www.vox.com/2014/7/2/5862026/sitting-vs-standing-health-risks-dangers
and the New York Times, from 2011:
http://www.nytimes.com/2011/04/17/magazine/mag-17sitting-t.html
The studies cited in these reports may be overturned or reinterpreted, as studies often are, but it seems clear that standing up every 20-30 minutes, even briefly, is simple good sense. If your school is forward-thinking on this issue, it will have at least some stand-up desks with high stools or chairs, so that students can alternately stand and sit as they like without disrupting the class in any way. If your school has no such desks, talk with your teachers and your principal and see what they say. With or without your school’s support, make a point of moving and changing positions throughout your day. You will feel better, and your brain will work better, if your body is kept in the best possible condition.
