How can you help your teenager become a better public speaker? You have heard me advise you numerous times to allow kids to speak up for themselves from a very young age, record them on their phones, and ultimately make them more comfortable and confident with their speaking skills from the beginning so they do not face any difficulties later in life.
But when they become teenagers, they might not want to do those things anymore. They may feel embarrassed in front of their parents. My advise is to try and get them to practice with some other friend, preferable in person, but they could be doing it through Skype or any other online connection. The key is to practice a presentation or report in front of another student, record it, and look at it. It does not necessarily have to be you.
As long as they are practicing with someone, they will go into that speech or presentation with a higher degree of confidence and will subsequently be better.