At the age of 15 still growing what is your height take balanced diet good mixture of carbohydrate
,Protein,and fats along with mironutrients vitamins and minerals
.if you are over weight or under weight ? if weight is less or BMI is less than 20 add more calories and proteins
. do more isometric and isotonic exercises to increase muscle mass. If you are overweight
reduce your calories. I would suggest ot try some outdoor sport and pay attention to your studies.