Find a group of friends who can play instruments. You should have a guitar player or two, a drummer, a bass player and a singer. If you don't have enough friends that play instruments, post flyers in the school cafeteria asking other students to try out for your band.
Come up with a band name. Since a band is a group effort, be sure you include everyone in the naming process. Everyone should be involved, or at least have an opinion about the name and logo for the band.
Find a place to practice. Perhaps one member of the band has a garage or one of the band member's parents will let you practice in the house during reasonable hours. You might even alternate between band members' houses and offer to help with chores for the privilege of being able to set up and practice.
Practice often. Weekends might be the most appropriate time. Don't let practice interfere with schoolwork.
Talk to the school music instructor when you're ready to play live. Maybe he can organize a concert for your band in front of the whole school. Ask the school principle about playing school dances. You won't get paid for it, but it's a good way to hone your live performance techniques.