I started exercising with the goal of losing weight. I exercise regularly five days a week for between 30 minutes and an hour. The majority of my work outs consist of running, with about an hour a week dedicated to weight training, as well as a little bit of cross training (elliptical, cycling, tennis, etc).
I have found that exercise is essential to both weight loss and weight maintenance. It will burn unwanted calories (because you can't have a perfect and strict diet 100% of the time) and it increases your metabolism so that you will burn more calories doing basic activities (even sitting in class). Exercise builds confidence, strength, and energy that can help you concentrate in school.
I haven't put a lot of focus into a certain type of diet, because those usually are not sustainable past a particular period of time. I calorie count, because at the end of the day, weight loss, gain, or maintenance are all determined by caloric intake. But I have also limited or eliminated foods that do not contribute to my overall health (sugary drinks, excess sweets, fast food), and started making smarter choices such as choosing whole grains over refined, choosing lean sources of protein (fish and chicken over red meat), and eating more fruit and vegetables. That's the type of "diet" that you can continue!