You can't practice. That adult is right. At your age, it is the stage of bone development, which will affect your normal development. If you want to lose weight, you can run. If the gym has a swimming pool, swimming+running would be great. It is ok to do sit-ups or something to practice abdominal muscles. When you are 20 years old and feel tall enough to lose weight, it's not too late to get in shape again. I have a high school classmate, who is fat before the first year of high school, thin after the third year of high school, and grows very fast. Being fat as a child was not a problem.