The majority of the HD requirements for OSX are in the OS itself, you might gain some space by ditching iMovie and Garage Band or whatever, but not that much. Picking an aggressive limit on your virtual machine's disk size might help. You'll probably have to shuffle files in and out of your VM a bit but it's better to have the guest OS keeping a hard limit than letting it spill over into your host's storage. Also depending on what your requirements for school are you might consider an older windows VM, XP's space requirements are significantly lower than 7's.
If none of that is applicable or sufficient then your only option seems to be shifting your VM off onto external storage. Kind of a pain, but at least it's possible to do.