I'm in grammar school, but I feel dumb & and afraid of my friends looking down upon me.?

Hey I'm a girl, and I'm in 5th grade of a grammar school, that's a school with subjects like Latin and ancient Greek, as well as the 'normal' subjects. I used to be really happy about being admitted to this school, but now I regret it a bit, sometimes. Some people around me, my friends, they are so intelligent, they get superhigh marks without even doing anything. One friend of mine went to participate in an international science-olympiad a few months ago, won silver, and another friend of mine, who is in 5th grade just like I am, is making his own robots and another one is doing research at the university. I'm doing nothing special. I like to shop, I love reading, writing, watching movies, painting, having fun with friends and family. I don't do any research, my marks at school are average. Sometimes I feel just so unworthy. My friends are fine, some of them are a little bit arrogant every now and then, and I'm afraid that they will think I'm stupid and dumb. Of course, they'll never say so, and they won't like themselves thinking that way I suppose, but still: sometimes I feel really shut out. I'm getting close to being insecure, and I used to be a very confident girl. Help!