You've got a lot going on. Just do the best you can.
Pre-making meals definitely sounds like a step in the right direction. Cooking most of my food on the weekends and negotiating with my roommate for dedicated kitchen time (he's like, the most time-inefficient cook ever) has really helped me out.
I have no idea about the specific benefits of yoga, but I do know that it's possible to do weight lifting with canned goods and such. Could you briskly walk around a large building on your campus? Does your campus have a gym for students? Do your school library or public library carry any exercise DVDs you think you could do?
Hang in there. You can do it!