Take 1 pudgy kid, tell him he's on a diet and needs to restrict calories. He doesn't eat breakfast. Eats small but junky lunch at school. He feels tired and unable to exercise, gets home from school binges, and falls into a food coma till dinner. Feed again. Go to bed. rinse, repeat. By going on a diet we've:
slammed the breaks on metabolism.
Reduced fruit and veggie intake (see study)
Piled all the calories into a short time span just before bed.
AAND the kid has no energy to get in the little exercise he was getting before the diet.
It wasn't just me.
See the ADA study.
Like everyone else teens need to be taught to change their diet rather than go on a diet. Restricting food is bad, it actually makes you fatter. It's a zero sum game. Eventually you are so starved you make bad choices, you feel guilty, you over compensate, and everything keeps spiraling until you give up.