Why does Shelly hold a grudge against Ben in the novel, You Go First?

It is revealed early on that Shelly is someone that Ben once told on for cheating on a test. And while this was the correct decision from a moral standpoint, Shelly continues to hate Ben straight into Middle School. This is shown through her bullying and willingness to humiliate him in front of the school. If Shelly had let go of her grudge, then its possible that Ben would actually have more friends, but because she is unwilling to do so, she isolates Ben from everyone around her.