I can understand how you feel. And that just because you are young, God cannot use you. But that is a lie from the enemy. I'll bet you did not know that you are helping a lot of people right now in this thread and in this site, and i'm sure in your daily life, you are a very helpful young man to your parents and friends and maybe even teachers.
When you have questions, that does help people. It causes them to get out their Bible, or Bible program and look things up, studying or renewing their minds to what God has said, and then God can build their faith, and build His presence in their life. Sometimes people just need that little push to get into the Word, and God takes over from there. And as people are seeking Him for what to say to you, God can speak to them about something else that He knows is going on in their life.
Sometimes people need to feel needed or that their life means something. And when they get to share with others, they feel like they come alive. They feel like they have a purpose.
Also another thing to remember is that how you behave or act at school, and in front of your friends, can be a testimony about God. How much He loves us and how great, wonderful and loving He is.
When you are kind and courteous to others, it warms their hearts. Maybe a teacher is having a bad day, and your grateful attitude, or smile or kindness...makes them feel as if they are making a difference in someones life. Teachers and parents, sometimes are the most unappreciated people in the whole world, or at least we feel like it sometimes. But as you show the love of Jesus to everyone...you are making a huge difference in their lives.
Trust me you (even at your age) have a purpose!!! Do know that God loves you and thinks you are valuable!!
Blessings