Posture Pals (1952)

A teacher gives her third-grade class a "posture test". Four students--two boys and two girls--fail it. The teacher then suggests that the four become each other's "posture pals" and point out to each other when they're slouching, slumping or engaged in other such deviant non-good-posture activities.

Unfortunately the movie Posture Pals is not yet available on HBO Max.