You sound like a nerd. Over-analysing everything. Think you have everything figured out. Even the first paragraph noting "mature posters only" just re-inforces my image that you are a nerd.
But we have hope for you yet.
1. Grades are still the only reason your at school. I would gladly be labelled a 'nerd' if i couldnt be cool and have good grades at the same time, luckily you can.
2. Change your image. Get a gym membership and get massive. Get contacts. Buy normal clothes. People often get labelled a nerd only because they look like one.
3. Start talking to people about *****, parties, funny stories, etc. Normal things.
4. And finally, reinforcing number 1. I'd rather be a nerd with good grades rather then a dumb fukken jock who will be working at the local supermarket until his 34.