Good grades are reinforcing to some children but not to others. someone explaining this fact from an early operant conditioning perspective would say that good grades are most likely to be reinforcers to children who:

Operant conditioning is a learning mechanism that consists on reinforcing certain behaviours through rewards or punishments respectively depending on whether the aim is to increase or decrease their frequency.

According to operant behaviour, good grades are most likely to be reinforcers to children who obtain a positive stimulus after the correct behaviour, for instance, feeling how they have overcome a challenge, see how proud their parents/teachers are, etc.