I will second that - getting a degree first is paramount for success. You have no idea how important a proper education will be, until you are too old to go back and change it. It will prepare you for life in general as well as pilot training. Flying is the easy part, building your character and backbone only come with experience. We have plenty of younglings with licenses, working dead-end jobs until they get their chance flying airplanes, desperate enough to take any ****ty deal that comes along, thinking it is god's gift.
Flying is amazing, and I recommend everyone to get at least a few trial lessons at some point in their life! But, like everything in life, it quickly becomes 'just another job' and plenty of pilots I know enjoy flying a small single engine prop around more than their regular airline job!
I started on gliders. Did you have a look into that? WAY more affordable than powered airplanes, teaches you very solid basic flying skills, and you will meet SO MANY people passionate about flying; From the old guy that started flying after retirement, to the military pilot that flies fast jets for a living, and everything in between. Talk to them, listen to their stories, and decide if you really want to do this professionally.
With regards to subjects. Don't let anyone tell you flying is for smart folks. You are 15 - You already surpassed the mathematics and physics level required to
absolutely ace the theory. Stick with the subjects you enjoy studying, and get a well balanced curriculum. No need to specialise in science if that is not your passion. Just do your best and enjoy what you're doing. As long as you don't end up getting a degree in basket weaving you will be fine. But, uni degrees are valuable in the long run, and will set you apart from the pack should you end up as a professional pilot.
Best of luck buddy, tough life choices ahead of you
