How can I get a better reputation?

I used to be a complete rebel at school. I'd skive off, not care about my grades, and break all the rules. I was constantly in detention and my parents were getting REALLY pissed off with me. Apparently I was hanging out with the 'wrong crowd'. Last term I decided to change and get good grades. I went from E's and F's to C's, B's, and one A. I was really impressed and happy, and my parents were over the moon! When I told my mates, some were really happy, but some started back-stabbing. They were saying things like 'You'll never get a good job, they'll just think you're a slut', 'The only thing you're good for is prostitution', and the worst one was 'You obviously cheated, you're thick as a plank - the only reason you're still in school is cause the teachers feel sorry for you!' I wrote all these down and showed the headteacher, but he said to ignore it. I want to get a good reputation, but I don't want to lose my mates - they're what help with my street cred. What should I do??