I had some time to kill so here's a long-winded answer.
+1 on DrockNJ's answer.
Having been in this game for awhile (over 30 years; I'm under 10% bodyfat and my deadlift is 450 in case "stats" matter) I'll give you this advice: modify your diet by improving your choices over time and by refining things as needed; don't try to be "perfect". Subway isn't perfect, but a grilled chicken sandwich, with some honey mustard (no cheese or mayo) is a
better choice a value meal, Big Mac, or other fast food. People who try to be perfect usually end up chucking it all because it's too frustrating, so make better choices, not perfect choices.
Another thing you can do is make your food over the weekend, freeze it, and then take is with you. Grill up a bunch of chicken. Freeze it. Put a piece in the fridge the night before a school day, throw it together for a sandwich in the AM and throw that in a cooler. Cheap, fast, and you control all of the ingredients. Can be done with tuna, turkey, etc. If you want to be more "hard core" (again, one thing at a time); throw in some pre-cooked brown rice or some cold veggies with a small amount of olive oil or dressing). Cottage cheese, an apple, some good quality nuts (walnuts, almonds, etc., in small amounts) can be thrown in the cooler as well. Voila. You have food! And it's going to be a lot cheaper in the long-run that Subway or other fast food.
As long as your diet is reasonable, and you "just have a small layer of belly fat" to lose making small changes in diet are probably all that's needed at your age. The other thing is your training plan. High intensity sprinting, interval training (cycle, stair climber, whatever), progressive circuit weight training type programs, tabata protocols, etc. yield much better results than long-duration, low intensity "cardio" or distance running. Need proof? Look at endurance athletes vs. sprinters. Who is more ripped? Endurance athletes are often kind of flabby. The "cut" guys and gals I see at the gym are those who engage in some form of high-intensity exercise while those on the hamster wheels look the same day after day month after month year after year. There's plenty out on the internets if you search. Simply Google "interval training". Oh, one last thing on training, whatever works works for awhile and then your body adapts. So mix things up!
Here are a few other tips that can be added in
over time that I've found can improve results:
Eat the majority of your carbs in the morning and after you work out. Note I didn't say "gorge" on carbs, but eating them at these times is optimal for fat loss and recovery.
Avoid carbs at night--any kind of carbs. A good dinner would be some grilled chicken or fish and some steamed veggies with a little bit of olive oil. (Doesn't apply to post workout nutrition....carbs are ok then.)
Don't avoid fats; use them in moderation but do avoid mixing fats with carbs. A bit of fish oil, if you can afford it, is a great thing for health and fat loss. 2-3 grams of fish oil is a wonderful thing.
A small amount of low fat cottage cheese as an after dinner snack is perfect as is any high-protein low carb low fat food.
Experiment, make small changes, test the results (bigger, leaner, fatter, whatever). Everyone is unique and cookie cutter approaches seldom work. (Oh, and things change as you age +1 or 2 years, so you need to adjust the diet as well.)