goody-goody; teacher's pet; goody two-shoes
Colloquial, often mildly derogatory term for someone who acts overly well-behaved or tries too hard to be seen as good, especially in school or social settings.
He's always trying to get in the teacher's good books, so his classmates treat him like a teacher's pet.
You don't have to act like such a goody-goody.
From いい子 (good child) plus the diminutive suffix ちゃん, used here to create a slightly mocking or childish tone.