How do the Tendos make money?

Akane and Ranma make so much of a mess.  Not to mention the guest they have.
I know that Nabiki is incharge of the money in the house hold but does she have a job?  or does she live off the money people owe her?

if the dojo is a teaching school, why dont we see any classes?
im sure if it was, there would be some sign of classes there or am i just mistaken?