I feel stupid because I have to work very hard for mediocre grades and I did poorly on an internet IQ test?

I know, It's not a real IQ test, just some silly thing someone set up all by themself probably. It's so depressing because my yonger siblings are getting better grades than me. My brother is in Jr High and made honner roll, I never got that in Jr High. My sister, who is in Elementary School, has always been praised for her intellengence and is under the impression that her intellect is far superior to mine. I'm in High School. I feel like being the oldest of my siblings slows me down.