[Reach]

You need to start lifting weights to build muscle.

Get a gym membership if your school doesn't have one and start lifting 3 times a week.

Focus on the the big compound lifts known as bench press, squat and deadlift initially. Start light and then slowly work on adding weight to the lifts.

You'll also have to start eating more at home in order to gain weight. I recommend picking up a protein supplement as well and take two scoops a day mixed with milk as a shake.

If you have any questions on details, programming, etc etc just ask away.

If you're as small as you say you are, putting on some mass will really boost your self esteem and also improve your overall health. Being small enough to blow away in the wind is no way to live.