Please advise if it's good for a teenager to lose weight by exercising and reducing calorie intake by half?

Depends: Certainly nothing wrong with exercising but i would be cautious about the calorie reduction. If the teen were eating far above average to start with, then a 50% reduction may be ok. But if the 50% reduction results in caloric intake far below daily baseline needs, then that would be a problem. See a physician for any further questions.