I'm an adult - with a child of my own.
The one thing i always noticed as a kid - was rock n roll always made me feel better about being me.
People ...like your parents or mine - made me feel like shit for being me.
They would never see what was actually wrong with me when i was going through hard times.. and often, I didn't let them see.
So often they would attack what stood out to them, and what they saw was that I was attracted to things like gangster rap, Marilyn Manson, Megadeth, punk rock, Janes Addiction or whatever. Things like that scared them, it shocked them - it was all flirting with thumbing it's nose at religion, at authority, with drug culture, with sex ..and with violence.
And they just figured they didn't have that stuff around when they were growing up - so that must be what's at fault. It's the descending influences from a exploitative and explicit media and entertainment landscape full of satanists, gays and drug users and gun toting black dudes.
The best thing you can do - is communicate to your parents whatever you real problems are that are getting in the way of your grades. Admit to yourself if you have toxic friends in your life that are adding to that stress and making life harder for you and cut them out. That's something i wish i could've done that would've greatly improved my grades at school.. i just surrounded myself with people who dragged me in the wrong direction and made turbulent teen years even more turbulent.
I've been there. You think you got it bad? My father bashed my stereo with a baseball bat. Why? Because i kept bringing explicit music into the house. I had Red Hot Chilli Pepper cds thrown away, NOFX cd's thrown away. Ice Cube and Megadeth cd's broken in screaming fits. Long talks about drugs b/c they found a Janes Addiction cd. Cypress Hill and Offspring cd's thrown in the trash.. THey got tired of throwing away my cd's and punishing me so he destroyed my stereo.
If you're stressed - try to find a positive outlet for that stress. Exercise, talk to someone. whatever.
And get your grades up. Study harder, ask for help, get a tutor if needed.
i'm 35. I never won my parents over that my musical tastes were somehow valid. And i never will. Partly b/c our views on politics and culture and god will never match up.
One day you'll be an adult with your own home and you can jam whatever the fuck you want, as loud as you want.
the best way you can win with your folks is to continue to like what you like and prove to them that music doesn't drag down your grades, your mental health or anything else.
All they're going to see is this scary shit they don't understand and don't want to understand - and would disagree with IF they understood..and they're going to see their kid liking that shit, and their kid struggling with stress and grades and correlate the two.
Prove them wrong.