I suggest you to concentrate more on your exams since it is on priority.
to loose weight you should reduce your total calorie intake & increase calorie burning.
due to your stress
of exams you may burn your calories but some are used to consume more food under stress so pl dont go for it.
consume high protein food
such has low fat milk, egg . panner fish etc.
be more on greens they boost your concentration , strength, & immunity.
consume 4-5 serving of fruits & vegetables in a day. which will help in getting good fibre & supress your hunger
avoid simple carbs such as rice, white bread etc.
hope it helps you