I think, instead of "demanding" my money back , I'd probably initiate a conversation with the shop owner about the issue and learn what she knows about the incident and plans to do about it, if anything.
Perhaps there were extenuating circumstances the class participants are not privy to.
Perhaps the instructor has planned to add an hour to the next class as well to make up for the first class missed.
Perhaps others have mentioned it to the owner/teacher and there is already a plan to compensate in the works.
Catch more flies with honey than vinegar.
Jan in VA