Probably sounds strange, but one of the singular pleasures I've had in flyfishing is teaching casting to newbies, mostly ages 8 thru 14, boys and girls, up to 5 at a time. Lots of fun if you can let go and just sit back and enjoy the whole process.
I try to keep learners joking around to keep them loose and having fun so they willingly follow 'suggestions'. Keep lessons short: two short stints on the lawn, before and after lunch, then down to the river the next day. The water is carefully chosen for clear back cast area, safe water, with lots of little dumb rainbows in it, diverse enough to allow better casters (read that as athletic and good listeners) onto faster water, and slower learners on the slow big pools where I can easily pick out knots, dodge flies, shadow cast with them, and take off fish. Oh yea, and water where no other ff can be bothered, as it is very frustrating to others. Generally, kids are so much easier to teach then adults; women are easier to teach then men (Women aren't expected to perform well, so they leave the ego out and just have fun with it). IMO, keep lessons(2) short, and spaced out so they have time to digest it: they will think about it on their own in between; make it fun. Some newbies get by with one lesson, most need 2 or more. Most courtesy breaches are really about ignorance, but not always. I have had beach fishing wrecked by someone throwing balls for his retriever into the water next to me, then when I sulk to the beach to wait the fool out, they want to chat me up about how much fishing they do! Go figure. A newby can learn from a patient mentor, some fools will never get it. Be patient with newbies on the water, but do tell them politely about breaches of fishing etiquette while explaining how better to fish that section, or while offering them a better fly pattern to use. It will have much more of an impact than an expletive.