I typically sort my frees into two categories - "trapped" frees, and frees. The trapped frees are where I can't go home, so I'm essentially stuck in school. I have about eleven of these in a fortnight.
I try to do as much homework as possible in my trapped frees, as I'm super lazy. Typically, this means I'm left with a couple of pieces to do left over, which can be done last-minute on a Sunday.
If you can bear to, stay after school. Not so much the really early finish, because it's nice to have a spare day, but are there any times when you don't have the final lesson of the day? If so, try to stick out the full day. In the day when you come in late, come in a little earlier to do some work. You don't need to utilise all of the non-trapped frees to their full extent, but a little extra time here and there will help you out no end, and you don't need to force yourself to do work when you're just too tired to work.