I don't understand what you are trying to say here. The way I read it, it seems like you are confused about how everyone gets their credits right now.
Why are you assuming class D "serves" or something? I think we are talking about different things here.
If Class D behaves correctly, they get money from the school/goverment,
If they don't behave they can get something from their upper classmen / or any class that didn't fail by begging or selling themselves, sure. But it's not like they have to spent money on them.
...
So I don't understand why you are saying the system wouldn't work if only Class A managed to get money.
The reason why Class D is hell is because it's unlikely that they'll get their shit together in the first place.