Hey. Have you tried writing or drawing (or something else) to keep yourself distracted? I honestly have no idea what could improve your grades, I'm in the same boat as you really. Its pretty hard to stay invested in something you dont care about. Finding an outlet and just going to town on it works for me though, maybe you could give it a try? just my personal experience~
And by the way, I would recommend going to see a psychologist, even if you dont want your parents to know.
Keeping it to yourself is just going to make it worst, trust me i know from personal experience. Try reaching out to a help phone. I don't know where you're from but just look up depression hotlines on the internet.