By the amount of students?
By the standard of students?
How about by the income of the owner? (Oooh that would be rude wouldn't it?)
What about the level of enjoyment of the owner (HOW do we measure THAT?)
Really the way you measure success is up to you. It is easy to be attracted to the admiration of other owners but at the end of the day, it is up to you to create YOUR business on YOUR terms.
The real trick is to connect the way you measure the performance of your staff with the way you measure the performance of the business. And to keep this visible.
Here is part one of a video that tackles the problem.