I don't think it's the sitting down so much, I think it's a lack of exercise and being sedentary in general which causes students to put on weight, including on the bum. So - do some exercise to keep the weight down. I've discovered cycling is also excellent for toning the behind.
A study also came out this week, widely reported in the media here, that watching lots of TV reduces your life span. 'Ha', I thought smugly, 'won't affect me, I'm virtuous and study all the time, and don't watch crap tv'. But no, turns out it's not watching TV per se - it's being sedentary. Can't win! Studying is good for warding off Alzheimers, now we find we lose anyway! So - move around, get up and take breaks at least.