Problem: Directors send unprepared students to region auditions each year, prolonging the audition process and frustrating the judges. This will not change because there is absolutely no way to have rules in place to limit the number of students a school can bring.
Solution: I suggest having a stopwatch for the monitor and auditions will be timed. Each student will get "X" time to audition after his warm up. When the time has expired, the student's audition time has ended and he must exit the room, period.
No, this will not stop the unprepared students from being there, but it will limit their time in the room. Actually, it is more fair than the way things are now because every student will get the same exact amount of time to audition. Right now, some take more time than others.
Or, forget my idea and go ahead and just keep doing what you're doing (the definition of insanity). Bottom line, want to solve the problem? Have time limits on the auditions.