What's happening here? It's a power struggle. Kids are already so powerless. They rarely have any control over their lives and they refuse to give up what little control they do have. So when Grace says STOP IT, Jim is thinking that if he does what he's told, now he's giving Grace power over him. That he's allowing Grace to be the boss of him. And he already has enough bosses (parents, teachers, older siblings, etc) and doesn't need any more. So there's a deliberate refusal to do what's asked so that he can prove (at least to himself) that he still has his power to make decisions for himself.