No one is born with great School/study skills. Everyone has to learn what works for them to get their desired grade.
For instance, I want top grades. So I try to figure out what has to be done when and at what pace to achieve that result. I have plenty of friends who are B and C level students. They do what they need to do to get those grades. I use a paper planner to pace myself and keep track of assignment due dates, but I know plenty of people that use free online services like Google calendar to keep track.
Try and be honest and realistic with yourself. If you struggle with writing, you will need more than two hours the night before the write a term paper.
It is time to start working on these skills. If you want to go to college or votech after high school, you will have to have these down. Stress is a huge trigger for mental illness flare ups. Waiting until the last minute to try and get something done or sweating if you passed a class is definitely stressful. Passing becomes a huge deal in college where failing can have immediate financial repercussions.
Even if higher Ed isn't in your plan, Learning to plan, organize, pace, accomplish tasks, and study is integral to surviving post high school in whatever you decide to do from waiting tables to brain surgery.