As someone who has trained youth and adults (just no at officiating), I agree that the instructor's method leaves much to be desired. That said, burn the letter!
I also would not apologize. I would however tell him something along the lines of; "Judging from you reaction the other night, I didn't do a good job of communicating with you. If my comments offended you, I will try to do a better job in the future. Thank you for your help. Hopefully we can chalk that encounter up to youthful inexperience on my part and move on without it damaging what I hope is a benefical relationship. Obviously, I have a lot to learn about officiating and someone with your experience can be a great source when it comes to advice and guidance."
If you really don't care for the guy, you may feel like you need a bath when you get home, but then a hot bath is a great way to relax.
Oh...say that in front of others. If they saw the incident, their opinion of you just may skyrocket.