Dumbness, friendship, school problem?!?

I'm not so clever and in in low sets for most of my cles. most of my friends are rather cleaver so i'm not in sets with them except for like 2/3 and sometimes they laugh if i fudge up a question and thats what we do because that's the kinda jokey sort of bestfriend type relationship we all have and i wouldn't mind it, but it's the fact that i am actually kinda dumb and i put my self down alot and when they laugh at me i actually get so upset? am i just being too much of a baby? i try my best in whatever i do at school and in exams i try so hard! and i think yes i've done well! i'm getting marks my mates have gotten a couple months back and they're now getting higher / EPs(exceptional performance) grades? any advice? it's tricky but i don't know what to do about it? do i forget it, if i try my best- thats all i can do and that's all that matters? or do i tell one of my mates? - i've told before but they always tell me i put myself down so much and prob thinks i do it for attention which i do not!:( any advice! pleasee