First, I think it is wonderful that you have it in your heart to want to share the gospel with your peers! As Paul said to Timothy: "Preach the word! Be ready in season and out of season." 2Timothy 4:2. As you are led by the Holy Spirit, He will open doors for you to share the love of God and salvation through Jesus with your school.
Assuming you live in the United States, there are areas of the country that are less restrictive regarding speaking openly about the Lord Jesus and the bible and some that are more restrictive; for example, rural Kentucky Public Schools versus Portland, Oregon Public Schools (generally speaking). Some public schools allow students to have bible studies before and after school on school property and have Christian clubs (such as Christian Athletes Association and Christian Fellowship). By all means, you should ask the Lord how to be involved in those types of organizations if they exist and if there are none, be bold to ask how you can start a club, prayer group, ect. If you attend a church, you may see how your church or youth group can become involved in your school: Some schools have an "inspirational speaker" come to speak to their sports team or school. You could ask your pastor or youth pastor if they would like to be involved in that type of activity. Also, ask your athletic director and principal how your church can become involved in the needs of your school (there are always needs such as family emergencies, illnesses, disasters, that arise within the families of students at your school and that is a great way for the church to partner with the school).
If you are talking about just evangelizing informally at your school (person-to-person, versus a group or organization), you will need to use wisdom (asking the Lord to lead your day-to-day life to witness to your peers), love others and do good to them (Jesus said in Mathew 5:16 "Let your light so shine before men that they see your good works and glorify your Father in heaven"). As you are led by the Holy Spirit, you will see doors open up to preach the gospel.
Here is a website that may help you with regard to knowing your rights as a student in the United States of America. (If your live in another country, you will need to find out what your rights in your area.) The ACLJ (the American Center for Law and Justice) is a Christian, legal organization that has many resources for finding out what your rights are as guaranteed in the Constitution of the United States of America:
God Bless You,