I'm in 12th grade and I have been getting C's in Maths. I studied tons and solved tons of exercises. Not enough?
I'd solved all the exercises as I could do to balance with having a bit of free time. It was enough.
But solving exercises doesn't work.
How should I study?
MSE is likely not the right place for such a question, and the answer will vary greatly with your own personality, which we don't know.
There are many unknowns here: what's the goal? Better grades I guess? But why? Do you want to study maths at a higher level, is it what you want or a requirement? Do you even enjoy maths? You seem to want free time: when I was in high school, maths was my free time. I don't want to criticize, I just try to understand how you feel about it.
If you have done tons of exercises, I assume you have the computational skills, but maybe do not see the big picture. Usually exercises are made to help see it, but maybe it's not enough here - and I don't think doing tons of exercises will help, I'd rather do a limited but carefully chosen set of exercises, that help me on specific areas I don't know very well yet. That's why the teacher is here usually : he will choose according to the needs of the classroom.
If you enjoy maths (and even if you don't), I suggest you try to tackle problems you find interesting and hard enough that the solution is not obvious. During your free time, why not? You could start with what is called recreational mathematics as this will teach you much about logic and how to think about problems and more generally about mathematics. You could also try problems from real life (could be applications of geometry and trigonometry, for instance, but not only that).
Apart from that, exercises are certainly a good thing, but you should also be able to understand the proofs. A good habit is to redo the proofs, and to try to change the hypotheses and see what breaks in the proof.
Just my two cents