Try writing down your feelings when you are upset, keep a dairy. We all react differently to stress, it is normal, but what we do can have consequences, so i suggest instead of reaching for food, try and expressing your feelings on paper, no matter how bad they are. Even going for a walk, another thing have a big glass of water when you feel like eating and you are not suppose to, that helps, often when we are hungry, we are really thirsty. What our body needs is water, and that will stop you feeling hungry. Drink lots of water. We all need to find something that relaxes us, when we are stressed or upset, i do not know you, so what do you like, do you like reading, it is great for unwinding. Walking is actually good for relaxation, who would have thought it, it actually relieves anxiety when feeling stressed.
My best suggestion to you is also talk to a friend when upset, or a sibling, if it is really bad talk to the school counselor. Otherwise write your feelings, it dos not matter how you get it out, it is a way of letting it go. All the best.