The girls' cut goes directly for the tuition. Unless Burke works the books and doesn't put money in the college, but in his pockets directly, forcing the college to work on a loss. That's a dumb thing to do. An IRS revision gets them into a load of trouble and possible jail time.
Burke definitely takes a bigger cut out of the profits as he's the one that actually does the fraud. Quinn also takes a cut, but apparently it's not big enough to help with her drug business. That's why she thought that if more people were involved, there was going to be more profits, but it eventually backfired.
I'm not sure if the girls get a cut from the profits. It's not mentioned anywhere, so they might be paid on the number of clients that they serve. It's definitely not for free though, as Camila mentioned multiple times that she has sex for money, even though it's not something that she's exactly proud of.