a goody-goody

goody-goody

1. noun Someone who exclusively follows the rules and caters to authority figures; a teacher's pet. Jill's classmates called her a goody-goody after she volunteered to supervise the class while the teacher was away.
2. adjective Self-righteous or sanctimonious. Forget your goody-goody rules and go out on a school night for once!
Farlex Dictionary of Idioms.

a ˈgoody-goody

,

a goody ˈtwo-shoes

(informal, disapproving) a person who behaves very well to please people in authority such as parents or teachers: Don’t be such a goody-goody! He’s a real goody two-shoes. He’d never do anything that might get him into trouble.
Farlex Partner Idioms Dictionary
See also:
  • goody-goody
  • goody
  • goody gumdrops
  • teacher's pet
  • a big tent
  • bummer
  • angle
  • angling
  • high
  • high, wide, and handsome
References in classic literature
Aren't you ashamed of being such a goody-goody, and of studying so hard?
Adventures Of Pinocchio
Similarly, a goody-goody teacher seems to be the unique professional--one with a fantastic figure--but empty-minded.
Goody-Goody