Obese teenagers benefit most from a combination of strength training and cardio
Severely overweight adolescents aged between 14 and 18 who want to lose weight and have enough will power to get up and exercise are best off combining strength training and cardio workouts. Canadian researchers at the University of Calgary believe that the combination leads to more fat loss than either strength training or cardio training alone.

In the Canadian study the researchers got fat teenagers to train four times a week for 22 weeks. They also put the adolescents on a diet that provided them with 250 kcal fewer than they burned.In total the researchers used 304 subjects, making it a large study. This is probably one of the reasons that the researchers were able to publish their findings in JAMA.
The researchers divided their subjects into four groups. One, the control group, ate less but did no exercise. Another group did strength training, yet another did cardio, and the fourth group did both kinds of training. All active groups also ate a reduced-calorie diet.
The combination group lost most fat: 1.7 kg. The cardio and strength training groups lost only 1.2 and 1.3 kg fat respectively. The subjects in the control group actually put on weight during the experiment.

The active group members' fat loss was mainly in the waist region, as the figure above shows.Conclusion
"Aerobic training, resistance training, and their combination decreased percentage body fat in obese adolescents", the Canadians conclude. "Combined aerobic and resistance exercise training tended to be superior to aerobic training alone in decreasing percentage body fat, waist circumference, and BMI."

"Adolescents who want to maximize the effect of exercise on these variables should ideally perform both aerobic and resistance exercise, but significant benefit can be achieved through either type of exercise alone."
Hmm... The Canadians' results are pretty pathetic. Almost six months of exercising daily and only 2 kg fat loss to show for it? These teens must have trained like wimps and not taken the diet seriously at all.
JAMA Pediatr. 2014 Nov;168(11):1006-14.