How to restrict printing by users

[Willi Eggeling]

Hi!
I'm currently building a unix based network in a german school. Every pupil has got an own username. they have to pay a certain fee to print a page. now i want to build a program which stores the amount a pupil has in a database and bind cups to that so cups looks into that db and checks if that user is allowed to print the number of pages he wants to print.

Do you have any ideas how to realize this?

Thanking you in anticipation

Willi Eggeling