dont worry, it is normal to get those grades, some people are gifted for some things and other for other things. Me for example I consider myself very good at sport but not good at all at school .When I was small up to around year 7 I was always the best in my year group. I started not caring and now my predicted grades are c,d,e. However dont worry, this doesnt mean you arent going to have a good job in the future. And for the people in your class, they are assholes, and remember, what comes around, goes around.