Let me first tell you: I know exactly where you're coming from.
Several months ago, I fell into a deep depression. During this time, I wanted to quit school (mind you I was less than halfway through my freshman year of high school!) so that I could travel the world, get a job, and essentially start living my life spiritually. I didn't fit in socially and felt that the curriculum was boring, useless, and took up all the time I could and should have been using to study Buddhism. I even considered running away from home because I was desperate to escape this life, this role that I had to play as a high school student who worked, worked, worked, and because of this (I believed) had no opportunities to practice dharma. Yes, I realized the danger and the stupidity of becoming a runaway, but I was convinced it was the only way my life could be worthwhile.
I feel like you're coming from this place as well... believing that quitting school now is the only way for you to have the life (as a monk) that you want. This really is not the case. You need an education. At the very least, you need a high school diploma. You're only 15, so you have a few more years to go. If you're an American and if you devote yourself, it is
possible to graduate by 16 (although you would only be receiving a high school education equivalency certificate, not a real diploma). I'm sure other countries have a program like this as well. Google "high school equivalency exam" for more information. In spite of this option, I wouldn't suggest it. High school can be a drag, but it's one of the best opportunities we have in life to grow as people. We develop skills in our upper school environments that we need for the rest of our lives. I am urging you to take full advantage of this. Also, while I understand and admire your commitment to becoming a monk, you are only 15. You have so much more to learn about yourself and about the world and I'd encourage you to stay open to following other paths. You may develop a new passion or passions in the years to come that you'd like to pursue as a career. Don't set your mind on one thing so soon, okay?
As for your parents not supporting your beliefs, this is very common. I'm the only Buddhist in my family and it drives me crazy sometimes but I have to accept it. Accept that your parents want what's best for you and may never fully understand your desire to devote your life to spiritual pursuits. Accept that they are flawed just as you are, but are doing the best they can with the awareness that they have just as you are. Do not fight them; love them and try to see things from their perspective.
I wish you all the luck on this path, dear. PM me if you ever want additional advice.
Peace comes from within. Do not seek it without. - The Buddha