Mona would have obviously had a way to pay for her tuition, but when offered a free tuition, for sex (which she enjoys), she jumped at the chance. Obviously it went sour when she found out she had to fuck old dudes, so she lost the free tuition and would haven fallen back to her initial method of payment.
Burke didn't delete her college account, he deleted her "scholarship". She didn't already have a scholarship before the suggestion of the free tuition, because that's what a scholarship is, part or fully paid tuition.
So the scholarship Burke deleted was her free tuition set up through Quinn, and when she reneged on the restaurant deal, Burke deleted the scholarship. That means that Quinn and Burke are in on this scheme, and it's likely a bogus scholarship that Burke has set up since he has access to college records.
But getting back to Mona, she was directly threatened by Quinn. So she left. Not just the HOTs, but B&R altogether. Perhaps she realised the gravity of the situation and wanted a clean slate.
Even Cammy thought Mona was going too far by having direct sex with clients (rather than anonymous sex through the glory holes).
After everything that Quinn said to Mona, Mona probably realised she'd made a huge mistake in the first place. Even if she didn't actually have sex with Burke, she was in a very uncomfortable position - she'd made enemies with Quinn and possibly a very high up official of the college (Burke).
I'd leave too rather than have that shit hanging over my head.