Unless your schoolmate is here, I don’t think you will get a definitive answer. And what people do on social media is not necessarily representative to what they would do in real life. So don’t take this block personally.

A mixup of names has been suggested a a possible explanation. It could just as well simply be a mistaken touch on the app or a misplaced click on the site. That person may have their own ghosts of school life past to fend off and they’ve chosen to make a clean break and not keep in touch with people from that time for their own mental health. Or they are friends with the people who used to bully you and doesn’t want them to spot you on their friends list. Or, while they treated you nicely and stood up for you, it’s still possible they just don’t like you regardless. There are many possible explanations.