Not all familes are going to be able to spend the extra time driving additional days.
I think the schedules should be ring-based. Here's an example:
4pm Private Dressage (Last name)
5pm Intro to Dressage (Sign up at X location)
6pm Private Dressage (Last name)
2pm Pretty Pony (Last name)
4pm Intro to Jumping (Sign up at x location)
5pm Private Jump (Last name)
This way, people would always know which arenas are in use and when. Group classes could be open on a signup, first-come first-served basis, with a limited waitlist. Make a rule that says you have to be IN the saddle at X minutes before your lesson (they should be warming up before the lesson anyway, right?), otherwise you forfeit your spot and a waitlist person can jump in *if they are tacked up and ready*. Try to lump similar things together - you don't want a jump course set up in the arena that's going to be used for Dressage all day and have to set up jumps elsewhere. If you have 4 arenas, there's no reason why 3 instructors should be lumped together unless there's just that many lessons. Lessons should not go on in the same arena if another arena is empty. Consider getting those arena microphones and give all the students earpieces, so the instructor can talk into the mic and the student will hear the instructor.