My university had 6 month prep courses in math and science for people who wanted math or science degrees, but didn't have the pre-requisite grades or had been out of high school for so long they'd forgotten it all. The course basically covers the last year and a half of high school and the first of half the first 'real' university level courses. If you passed the prep course they'd let you in. Surely you could do something like the for CS as well.