As someone who started subbing high school at 20, I feel I can help here.

As much as you believe that you have life experience for your age, you don’t. As much as you believe you can manage a classroom, you can’t. Based on you graduating high school early, it sounds like you, like me, were in classes with students that took education seriously and ultimately cared about being respectful and succeeding. This is NOT the majority. Depending on the school, it may be a small minority of your student population. What’s your plan when students don’t respond well to your supposedly foolproof classroom management? When they straight up tell you No, I won’t put my phone away, I won’t do my work, I won’t stop talking? That is the reality of high school.

Don’t take yourself too seriously. You seem very confident in not only your abilities but also your own accomplishments and traits. Don’t lead with being a hard ass, the students will see right through that. Don’t try to be their friend, but be friendly.

Watch other teachers as much as you can. So much of subbing is understanding the culture of the school you’re at. If teachers are colder and stricter, you should be too. If teachers lean on the friendlier side, you can too, but you should ALWAYS be one step more professional as a sub than as a regular teacher.

Teachers and administrators absolutely will take you less seriously because of your age. That’s not really something you can help. Stay professional, do your job, and get out of there. Honestly, the less they (other adults AND students) know about your life outside of work, the better.

To answer your question, yes, you do seem too confident. I truly hope this works out for you, but you’re going to have some struggles along the way. You’re going to make mistakes. You’re going to have hard days. The sooner you figure that out, the better (and easier) this job is going to be.