I had the option of applying as either a mature student or using my highschool marks as the basis for my application. I ended up using my HS marks to get into school.
The boy that I dated from yoga class was a mature student. To apply he had to show his HS marks, which were in the high 60's, he also had to write an essay (just a page or two) explaining how his life experience would help him in university. He got in no problem, he had to take a couple of extra classes his first year (university 100, and 110) which basically are about note-taking, paper-writing and basic academic skills. He also has to meet with an academic adviser every semester and his marks have to stay above a 65% I believe or he is on academic probation.
This is for UofR, but I'm sure it's pretty similar in other institutions. Good on you for going back to school! good luck
