You're already way ahead of most skiers simply by planning to tip your instructor. I always tip at the end of multi-day instruction. Once you set a daily rate, it looks funny to change it even though your experience may change. A while back, I had 3 days with an instructor in which each day exceeded the previous by leaps and bounds. I always tip generously, but I was so pleased that I tipped twice what I had originally intended to for the 3 days. Very happy student at end of last day = smiling instructor. Waiting until the end gives you the flexibility to dynamically adjust without day to day irregularity. I've also noticed at week-long ski camps that tips are generally given at the end of the week.
I'm not an instructor, but maybe this suggestion will make sense anyway. If you feel awkward, consider that you are likely to see these individuals again in a non-exam clinic setting some day. Why not give a cumulative thank you at the later date? Besides the monetary tip you could tell them about the effect they've had on your teaching and skiing and how grateful you've been for their guidance -- feed both their wallet and their sense of professional accomplishment.