I have slacked off working out this past year, but freshman year I'd try to workout on days when I'm not busy. Between Saturday and Sunday ( So, no class), I'd work out, so that leaves one or two days during the week ( I generally don't work out more than 4X a week).
I try to work out when I don't have anything else. If Monday-Friday you do volunteer work two days , then workout out during the other two or three days. If one one of those days you have 4 hour orgo lab, work out on the remaining two days.
I also try to workout before studying. So class is out, you eat , you hit the gym,
then sit down with your books. The other way around, you may decide you are too tired and you may not see working out as as mandatory as studying.
Hope these tips helped

PS- I did get back in shape this summer, though. Hopefully will workout out enough during junior year.