Hi Selina
In a month you can lose 2-4 kg - it is a healthy weight loss. Healthy weight loss can do by combination of balanced diet & exercise, life style modification.
In 14yr old is age to enjoy ur diet, life but maintain ur limits & enjoy.
Diet modification : Include more fibers foods like fruits, vegetables, whole grains, etc. Avoid deep fried foods, junk foods, etc.
Treat ur self weekly once but maintain ur qty.
Eat 4-5 small meals with healthy snacks.
Exercise : go for any sports like tennis, volley ball, thrown ball, basket ball for 1 hr daily.
Avoid coffee, tea- try drink skimmed milk.
By taking healthy foods, vegetables, fruits, whole grains which improves brain & gives good concentration to ur studies.