The first piece of business is to make sure that you have permission to include anyone under 18 in an exercise program and that they have no issues that would exclude them from exercise. A parent or guardian can sign a waiver and provide medical clearance.
After the teens are cleared to exercise, I would first ask if there is a type of exercise that they would like to do. I am assuming that you are not going to make the exercise mandatory. So you will have better involvement if you let them have a voice in the sessons. And make it fun. Break all the movements into small parts to start and then put them together as the teens get it.
You may want to try to bring in a voluntary instructor if you don’t have a lot of experience. And if you are an instructor, let the teens know what type of exercise you teach and ask if any of them are interested in participating.
There are a lot options. If you have interest, but not sure where to go from there, come back in and ask for more help.