When considering tipping ski instructors, think of his or her service to you this way:
Do you tip a waiter or waitress? They brought you the food that someone else prepared. They were kind and attentive and made your meal more pleasant and complete.
Your ski instructor prepared many years to teach you. Then when he or she introduced himself or herself to you, he or she tailored his or her teaching plan specifically to your current abilities, your stated goals, with his or her expert observation of your objective needs.
And whether or not it is apparent to you, he or she selected what, in his or her opinion, is right terrain for you to progress. He or she kept you in the safest place on the run. Helps you up when you fall, corrects your mistakes and encourages your progress.
Because he or she is a "teacher" he or she has been fingerprinted by the State Department of Justice. (Which costs the instructor money.)
If your instructor is a certified instructor, he or she also pays annual dues to the certifying organization (such as PSIA - Professional Ski Instructors of America) (~$135/yr.).
And then on his own time and at his or her own expense, your instructor has purchased and studied the books and watched the videos mandated by the certifying organization (~$100), taken clinics to develop his or her skiing and teaching skills ($125-$250), then taken an exam prep course ($100/day plus $15) to prepare for each certification exam, then taken the multi-day exam ($~$255-265).
psia-w.org
Then once certified, to maintain his or her certification your instructor must complete Mandatory Clinics/Education at an additional unreimbursed expense of ~$125-$250 per year.
Then there's the ski equipment and non-uniform apparel, and the vehicle to get to the resort to teach you on the days that he or she is scheduled to teach despite all manner of inclement weather and resulting road closures, collisions and conditions and, last but not least, the cost of the gasoline to do so.
Your instructor chose to pay all of this out of his or her pocket.
All for less than $25/hour.
You don't know this.
Your instructor will never tell you.
Your instructor teaches you out of love.
Love for the sport and love for you and others who want to learn the sport.
To move without friction in balance like a bird balancing on the wind.
To introduce you to and to share with you the joy of skiing.