goody-goody 
informal
noun
A smug or ostentatiously virtuous person.Kaitlin had been one of the goody-goodies in school....
- They weren't all insufferable goody-goodies as children, or always models of perfect sanctity as adults.
- If the teacher ever leaves the room and puts a few goody-goodies in charge, look out.
adjective
Smug or ostentatiously virtuous.I have some goody-goody friends who are really nice, but we don't have anything in common....
- Maybe they just like goody-goody sweet entertainment.
- At that time, their denial was their belief that they were popular all over the World as a paternalistic big brotherly, goody-goody neighbor.
Synonyms
self-righteous, sanctimonious, pious;
prim and proper, prim, proper, strait-laced, prudish, priggish, puritanical, moralistic, prissy, mimsy, niminy-piminy, shockable, Victorian, old-maidish, schoolmistressy, schoolmarmish, governessy
informal starchy, square, fuddy-duddy, stick-in-the-mud
rare Grundyish, Pecksniffian