What does "on a curve" mean? In what circumstances could I use it?

Native speaker’s answer
Rebecca
The whole phrase here is "graded on a curve." "On a curve" by itself is not a common phrase! To "grade on a curve" means to adjust the results of an assignment or test to meet the teacher's desired range of scores. It usually boosts the lower scores to higher scores. So here, she's saying that she did extra credit for no reason since the grades were pushed up afterwards. Ex: The class was graded on a curve, so they all passed. Ex: I hope the teachers decide to grade on a curve.