Felling left out by my best friends...?

Okay so here's the thing, I kind of feel like the "third wheel" when with my two best friends. I kind of feel left out and not wanted. Like, I'm the quite one standing behind them. They're really different from me. They're both athletic and a little more popular than me. And well recently, it seems like they don't want me around at school. What do you think I should do? :/