Well, are you sure you won’t be able to use your school gym?
If gyms and other equipment are absolutely unavailable, then there are still a couple of things you can do at home. All you need is a backpack, duffel bag(or anything that can accommodate a good amount of stuff while you can still carry it), and a pull-up bar.
The exercises you want to do are-
Squat (or if you can do it pistol squats)
A whole bunch of other bw/BBless exercises.
Use the backpack and duffel bag for additional weight as doing pure bw exercises get easier. For example, put a bunch of books into your backpack and wear it while you do push-ups and pull/chin-ups. General rule of the thumb is that you can start using additional weight if you can do any particular exercise for more than 10-12 reps without issue.
If you can do handstands, then handstand pushups are a superb way to strengthen your shoulders.
A really simple template would be to just split the exercises you want to do into two separate piles and alternate them daily.
Say you want to do push-up/pull/chin-up/some form of shoulder exercise/squat/direct core exercise. You would just split them into an A/B pile and do A on Mon, B on Tues, and so on and so forth. Start on a number of reps that feels easy for you, and then just add another rep every week.
There’s tons of ways to program bw exercises. Because they’re nowhere near as intensive on your body as barbell lifts, you can do them much more frequently as long as you listen to your body.
Just keep in mind that all lifts are basically you doing some certain movement while placing some form of load to make it harder. So it’s not necessary to have a barbell or dumbbell to do lifts. Figure out a way to make things work.
All of these will make you stronger and more muscular if you do them consistently. They won’t make you particularly bigger though. I would discourage you from trying out any heavy calorie surplus diet. However, since you’re 15 you should be eating a lot of food. Just eat till you’re full, and try not to eat absolute junk food like ice cream often. They’ll fill you up and won’t be all that useful in terms of nutrition.
Look to run as well. Run a lot. There’s a reason why sprinters look like absolute monsters.