Well maybe you're not putting in enough effort.
When you've been getting bad marks for a while, you have to put in a tremendous amount of effort to become 'average' but if you keep at it you will eventually become quite good. Also, on the plus side you get to learn the meaning of hard work and you become more motivated to succeed.
I struggled a lot at school, but I worked extra hard and even though I didn't get any rewards while at school (I even failed senior maths), now that I'm in college, I'm doing really well and I don't work anywhere near as hard. I'm getting easy 80% for mathematics with minimal work and I've gotten distinctions for over half of my subjects.
I have a friend who was in the top 2% of the state academically at school but he never worked hard and when he got to Uni, he didn't have the motivation to get the work done so he barely passed his courses.
School grades don't mean shit. It's the work that you put in that's important.