I attend a martial arts club with an unusually high number of teenagers. Of about thirty students, there are approximately three adults, and twenty-seven teenagers.
Actually the teaching methods that I will describe here should apply to people of all ages, however you'll notice their effect the most when teaching teenagers, as they tend to be more honest about whether or not they're bored, and how much longer their attention span lasts.
Rule number one: never patronize young people. They can tell immediately if you're patronizing them, because they're used to it at school. If you individually want them to stop listening, then patronizing will work, otherwise always avoid. It's particularly annoying in martial arts, where age does not make a difference in determining someone's rank. It's quite possible to have a twelve-year-old black belt. Just because they're twelve, does not mean they're any less of a black belt. Do not condescend, marginalise, or alienate young people. They just respond differently to bad teaching.
Rule number two: if you do not sound interested in what you're teaching, there's no chance anyone else will be either. There are two solutions to this, either feign an interest in what you're teaching, or stop teaching that and look at something you're really interested in. I would advise the second of those options, as in doing so, you will probably increase the diversity of knowledge in your class.
Rule number three: do not lie. Quite often as instructors we will pretended that something is very important when we know it's not, and your students know it's not. Always be honest about what you're teaching and why you're teaching it. If you do not you will be ferreted out instantly, and nobody will listen.
Rule number four: give correct feedback. Young people can always tell if you're giving generic feedback. If something is truly unimpressive, you know it, and the student knows it, so do not say that it was good. Many people think it's impolite to tell someone that what they did was terrible. No. That's not going to work. The only way that your positive comments have any value or impact is if you also give out negative ones. If students know that it's hard to get you to say that something is good, then they'll be especially pleased when you do.
Rule number five: give students responsibility. This rule does apply more to young people than adults. Young people are incredibly dependable, probably because they have not entered the world of work yet. If you have a good student, then give them some responsibility, and they usually respond incredibly well. If they then abuse that responsibility, you must take it away again. Be harsh and be honest. I have always been surprised at how well teens can teach themselves, even if a particular student strikes me as inattentive.