Do you know for a fact that people think you are mentally retarded or are you making assumptions?
Also, if you are comfortable sharing, what are some things you do that are considered "weird"?
I go to a very preppy high school and I do notice that a lot of high school kids are too afraid to do anything that they don't see their peers doing, so there isn't a lot of room for individuality. For example, many of my peers make fun of people who play chess because of the negative stereotypes associated with it, despite chess being a completely normal hobby.
In a way, I sort of feel for you. I used to be really quiet, as well. I used to hear people talking about me behind my back about how annoying it was that I was so shy. I guess I kind of branched out a bit as I got older and now I'm actually pretty talkative and bubbly for an introvert. Most people are able to overcome their awkwardness as they get older, but sometimes it takes a lot longer because you have to wait until you find yourself in a situation that you are comfortable in. I'm sure college will be a lot easier for you, granted you don't go to a "social school" like USC.