Get them excited about the work. Get them involved in doing real work. Work they can see is relevant to life, and in particular, their lives.
You’re the teacher. You are responsible for demonstrating a sense of enthusiasm. Reward and punishment, while they work, don’t have a lasting impact. You want your students to love whatever it is you are teaching. Or at least think it’s cool.
Kids want to matter. So many teenagers feel like there is no place for them in society. Society values them little. It throws them into schools when they want to be out making a contribution already. It gives them shit jobs when they think they can do so much more. They have so much energy. A good teacher can just channel that energy into interest towards whatever it is they are supposed to teach.
A good teacher learns more from the students than the students learn from the teacher. How can you learn from students? You have to put them in in a position where what they know from life matters. They need to solve real problems, and it can’t be problems that have a set answer.
Of course, this is the same stuff as you do for adult learners or, for that matter, elementary school kids. Unfortunately, most educational administrative systems don’t leave teachers free to do this. For it to work, you kind of have to let go of all the forms of accountability the school system imposes. Tests and grades don’t really matter. Learning does.
You gotta pull up your sleeves, and pitch in, working with the kids like you are all on the same team. You’re the consultant. They do the work, and when they get in trouble, you help them figure it out. Let me tell you. Teaching is so much easier when they do all the work. You just stand aside and try not to get in their way.
Edit—if you do it right, you can have the students hold each other accountable. That way you don’t have to face the issue of letting them fail or not letting them fail. The class will do that for you. If you want to be really radical, you can let them have input on evaluating each others work. I wouldn’t tell them they are evaluating it officially, but I would have them critique work in a problem-solving kind of way. You’re looking at process as much as result. It’s like the fish thingy. Teach them process, and they can learn forever. Teach them a fact, and all they will know is the fact.