There was a time when getting spanked for bad grades was the norm. We didn't think about alternative methods of discipline because it was a known fact that spanking curbs undesirable behavior.
Having an occasional C or even D might simply mean that a particular subject needs more attention in the area of tutoring. However, when 3 or more C's begin to show up in classes where A's and B's are the norm then it usually means someone isn't paying attention or caring about school. That someone needs her adult bottom bared and taught a sound lesson.
Once that good ol' fashion spanking is delivered I find that the grades start going back up because she's just been slacking off. If she hasn't been slacking off then I discovered that she remembers that spanking and asks for help when the grades start slipping.