Well, of course the profession is not entirely standardized, although the RID is trying to work toward that. There are standard practice papers on their web site and also a Code of Professional Conduct that give principles and guidelines for the professional interpreter.
I was just trying to say that an interpreter and the college should have some kind of agreement set up, whether it be an hourly one, or by the course, or whatever. I can't speak to your situation, but if a professor cancels a class with less than 24-hours' notice here, it's understood that the interpreter has the right to charge for that block of time.
Also, interpreters usually charge portal-to-portal, meaning their drive time; that whole block of time gets charged to the client. Many maintenance contractors who make housecalls (ie, plumbers, electricians) charge portal-to-portal as well.
Those are standard practices in the PA-NJ-NY area at least.