I didn't pay tax or tip for my lessons, but that was 25 years ago.
You should NOT pay tax; you are being provided a service and services are not taxable. I can maybe, kinda, if I stretch my mind really far, see how maybe a tip could be given IF one feels the service has been exemplary, but I wouldn't like to be told the gratuity is mandatory.
See if you can get your money refunded by Groupon. Its worth a shot.