If what I am tipping is close to twice the instructors hourly wage I don't see that it is a 'tip'. More like the ski school is saying 'hey, it's $800 a day if you and your family would like to learn to ski' and oh by the way, you need to hire an instructor as well. But we will leave that as little grey area and maybe you will pay them adequately and maybe not.
Ski schools are operating without competition. If there was competition you would see prices regulate and instructors would start to get paid according to their qualifications and overall worth. This doesn't seem to be the case.
You cannot argue that a Resort has to pay more to operate the lessons than they do the whole resort, in fact, it is in their interest to make reasonably priced lessons available to improve everyone on the mountain's enjoyment, safety and skill, whether novice or expert. I don't see that they are ploughing the money they charge into education, training, great locker rooms, staff meals or accommodation.
I don't see that the schools provide any special infrastructure for ski lessons besides the kids shelter and the flag where you meet.
A priority queue would exist if there were no ski instructors, because people would pay for it. Your lift ticket should and does cover the cost of you skiing. You having a lesson doesn't mean you use more snow/lift/carpark or cafe facilities. All of which you already pay for on a user pays system.
The advertising that you talk about for the ski school is on the resort website, just one page of usually a very simple web site, mostly you can't even book lessons on line, you have to phone or turn up. I have never seen any targetted advertising for ski lessons separate to resort advertising.
I think all instructors in the ski school should work as contractors with their resume up on the internet or on the wall. To book them you do it through the ski school who takes a reasonable cut - just like an employment agency does,which covers their admin, infrastructure and staff costs plus a profit. The ski school could provide professional training etc, but they should not be charging what they currently do for the lessons regardless of the experience or ability of the instructor. I see so many posts about schools hiring low or no experienced instructors and it is true, I have often taken lessons one day with people who have experience, the next day none and a completely different way of teaching. Make it a free market, the guys put up their resumes and their costs and then we choose. The ski school takes a per day fee from the contractor when booked or a percentage. If they want to hire out their own instructors let them go head to head with the contractors.
Resorts pay a fee to lease huge tracts of public lands and they charge the public to come onto and use that land, fair enough.
Then when I get good service from an instructor I can pay a tip or gratuity that says thanks, have a beer or a meal on me, but please don't be relying on me to pay your mortgage or put your kids through school with my tip. Right now the ski schools are charging outrageous amounts and paying their staff a pittance. I don't think an audit would show up that this is what they need to charge to make ends meet.