They're both important. If I'm an HR worker filtering through a thousand resumés (even more than that is probably realistic), who is getting considered first? Someone with no experience and no schooling, or someone with no experience and a university degree? (And that's not including all the people like me who have a university degree (or two or three), and 3+ years of experience in the field).

If you're not able to get yourself motivated enough to suceed in high school (which, not to be an asshole, but high school is a joke compared to university), how are you going to teach yourself to program games (which is far beyond anything you have to do in high school).

With that said, it's not impossible to make games without a degree (although I've never met a PROGRAMMER without some sort of related post secondary certificate or diploma, I have met people from other disciplines without a degree, but they worked their asses off to get where they were). But, I'm willing to bet the ones that succeed on this path aren't the ones whose grades "weren't good enough for university" (more often than not anyways).

Why not just work harder at school and give yourself a better chance?