Hello,
Welcome to ehealthforum and thanks for writing
I would not recommend you at all to just be on apples.You are in a growing stage and require all nutrients from different types of food.Just diesting is not a solution for weight loss as even if the weight is lost it is regained back.You need to have a balanced diet along with exercises as jogging,cycling,swimming etc.
You can also consider to join a Gym centre where under a trainer you can do exercises suited best to you.Not eating properly will definitely cause loss of energy and other issues as lack of nutrients would effect the thinking and concentrating ability which can also affect the studies.
Kindly start exercising and consult a Nutritionist who can plan your meals according to your choice and also follow you up.
Take care!