Most of my friends at school do not know Jesus. What do I do?
Well, first you must always be careful when your friends are not Christian. Their direction in life doesn't match, or shouldn't match yours. They will want to do things that you should not do. Paul warns of this in 2 Cor. 6:14. Anytime you play around with fire, you can get burned. This is a common expression that is directly applicable.
On the other hand, our command is to preach the gospel. You can't do that if you never interact with anyone. Jesus had dinner with and spoke to many people for which the religious leaders condemned Him.
My best advice is to pay attention to the prompting of the Holy Spirit. Try to live as you think Jesus would and do what you think Jesus would do. The Holy Spirit will guide you. They are watching what you do, listening to what comes out of your mouth, judging what you think is funny or sad, listening to what excites you, observing every moment of your public life. You are witnessing to people whether you know it or not.
If the Holy Spirit prompts you to mention God to them or talk to them about why you think something is wrong, you should do it. Hammering everyone with the Gospel Message won't make many converts. But if God has prepared a heart and you see God wanting to move in a particular area, move with Him. The results might amaze you.
The Gospel Message link has a section with some common questions you might be asked or common excuses you might hear. If you memorize some of the corresponding scriptures or bookmark it on your phone, you'll be ready to answer with Biblical truth against the lies of the enemy.