It’s Because He’s Dumb
There are some people who feel intimidated by their partners if they exhibit greater levels of intelligence than them. We call those people “insecure,” and they really need to grow up. Sure, your GPA is lower than your SO’s, but in the long run, why does that matter?
Well, it mattered enough to the OP’s ex-boyfriend that he dumped her when their GCSE scores were announced. He had already proved to the school that he wasn’t as smart as her, then he proved to the world that he’s a doofus by breaking up with her. She posted her breakup story online and we all got a hearty laugh from his foolish way of coping with his lack of intelligence.