The reunion takes place 10-20 years since the group was split up and the place is often a small town where everybody knows each other.
For this character to exist we also need the popular girl, or preferably the entire popular girl gang. They left town for a big city long ago and has only been back in town a few times since. The nerd has always had a crush on one of them, and when he sees her now after so many yeard he approaches her and starts a conversation. He tries to remind her who he is, but nothing rings a bell. Of course he used to sit behind her in class and maybe even helped her with some homework occasionally, but that's not what eventually will remind her who he is. Usually, there is a silly name they used to call him or something embarrassing he did once that makes her remember him. Finally, at the end of the movie, the two will become a couple. Sorry for the spoiler.
I know this is supposed to be one of the top jokes in these movies, but it has honestly never been funny.