I wouldn't make a big issue of it. I know it's uncomfortable, and it's totally inappropriate and unprofessional on the part of the principal. However, your goal is not to be the principal's friend. Don't be outwardly hostile to him - just smile and wave, and be the bigger person.
I had a similar issue with a teacher's aid at my son's school. She was never overly friendly, but at least seemed efficient and professional. After half a dozen meetings with the teacher and the aide, she eventually had nothing nice to say to me, and even went so far to point out every tiny deficit my son had. (She once spent ten minutes lecturing me after school because my son was caught reading a book under his desk during class - a problem, to be sure, but not serious enough to justify keeping me standing there talking about it when my kids were late for their music lesson). I always got the impression from her that she thought I didn't realize my son was not perfect, and felt the need to prove it to me.
It's possible that the principal doesn't see a need for differentiated education for your child, and sees your "interference" as simply getting in the way of the daily routine. I doubt you will change his mind about that - and honestly, it's not worth the effort to try.
