A goody-goody is someone who tries extra hard to be well-behaved, helpful, and “good,” mostly to please parents, teachers, or other grown-ups. Children use it most, and they almost never mean it kindly. It is a teasing, slightly sour word: the speaker thinks that person is showing off how good they are, or fishing for favour, not simply being decent. You hear it in the playground more than in a classroom essay. Adults can use it too, in a light, mocking way, about someone who always sits up straight and never breaks a small rule. It is informal. Though actually, the person may not be pretending at all — they may just like rules — but the word still paints them as too eager to please.