Yes and I think that is the way it works if the shop schedules the class. if the instructor schedules the class like i often see here then the shop takes a cut for pool and class space in combination with hopes of selling a bunch of gear to boot. I think the key to this is who is sponsoring the class. Its a ballance of does the shop need the instructor or the instructor needing a place to work from. You use of the words "classes the shop did not normally offer" very likely prevails.
What you see here is part of the legal problems associated with scuba instruction in the US. With that first shop, we were classified as contract workers. With the shop scheduling the classes (and other such issues), however,
we were legally employees. This came to a head was when we discovered that different instructors were being paid at different rates. When we asked why, we learned that the ones who were being paid at the higher rate also did other work for the store (like retail sales) and were therefore employees. As employees, they had to be paid at the higher rate legally. As it turned out, all the instructors working for the store were women, and all the contract instructors were men, so the shop (managed by a woman) was paying all the women more than all the men (except her partner, who also did work for the store). When this threatened to blow up, the shop realized they were on shaky legal ground and changed the pay system, making everyone an employee (which is what they legally were all along).
The second shop also made instructors employees, but with its system of paying per pupil, instructors teaching low enrollment classes (like my specialty and tech classes) quite often made less than minimum age--sometimes far less. When I pointed this out to the director of instruction, he showed his knowledge of labor law by saying that it was legal to pay less than minimum wage in a profession where the employee might get tips--which for us might happen once every couple of months. He used wait staff as an example, and he was quite dubious when I told him that if wait staff do not get minimum wage with their added tips, management is legally required to make up the difference. He was not moved by that argument, and it is one of the reasons I don't work there anymore.
In summary, I think that if the IRS randomly checked how dive shops pay employees, there would be a whole lot of illegalities found across the country.