Here's my non-horsey opinion.
I'd love to find a teenager who was able and willing to teach me to ride on her own horse, hopefully at a lesser price that certified instructors. I'd think of it as providing a teenager with experience, and I wouldn't expect perfection. However, if you were to tell me to just get on the horse and wing it, that wouldn't work either.
I've observed great teenage instructors in various sports, and I taught tennis from the age of 15. However, since I learned to play tennis with a qualified instructor, I knew very well what I had to do in order to teach. I had lesson plans and strategies ready. I was actually sought after, because adults and children loved my lessons.
If I were you, I'd watch beginner lessons--first on youtube, then go to different barns. Read books on how to teach riding. There are different ways, I'm sure, and different approaches, and after some research you'll find something that works for you.
Don't overestimate yourself and your horse. Keep your student safe. Make sure you explore the liability issue and that your parents are in agreement.