I suspect your 15-year old self would tell you to shut the fuck up before you even got to the Vice Principal part.Be damn grateful and give Mom and Dad some credit, especially Dad. He put his life on the line to make sure you lived a comfortable one and Mom's still sacrificing a lot to this day.
However, don't listen to everything Mom and Dad say, especially Mom. They still think Mainstream Media is reliable through social media and believe everything on YouTube. Through that, they will become even more paranoid than before.
Don't feel sorry for your Grandmother when she dies. She was a horrible person and the main reason why you always felt like shit. Not to mention, she also was the reason why you and your sister never got along when you were younger.
Talk to your younger brother more and connect with him. You two will barely talk in the future.
Friends come and go, but almost all your friends will end up being virtue-signaling assholes, lazy do-nothings, or wannabe Instagram models with an OnlyFans. Best to keep to yourself and keep an eye on your enemies. One of them is the vice-principal, who is later exposed as a moonlighting stripper.
That crush you had in high school will go full cuck mode in a couple of years. Best to stop swooning over him. The other one will be charged for aggravated assault, so you dodged a bullet there.
Stop using Tumblr and Twitter. Both sites are a living hell and it will only get worse over time. Start using alternative sites, stick to forum sites, and start archiving everything.
Start that online website early and keep posting. You're gonna need to make one in a couple of years.
Get a part-time job and save some money. Learn some financial skills early and always ask Dad for financial advice before you start buying useless shit.
Stop saying that you're gonna kill yourself. You're on this planet for a reason (and you're too much of a pussy to do it anyway).
Don't beat yourself up over everything. Things happen, let it go, it's all in the past.
Keep a copy of that comic book you made in art class. You're going to lose it and it's going to get on your nerves for the rest of your life.
Keep your head up, speak up more, and be honest about it. You'd be surprised how many people would like to hear your views on things.
Always ask questions, even if they sound stupid. Don't put your whole trust in someone just because they have "authority" over you.
Read more classic novels, especially reread Brave New World and 1984 ('cause you'll be living in it in a couple of years).
When there's a good opportunity, fucking take it. Stop second-guessing and preempting yourself and others.
GET THE FUCK OUT OF YOUR BED AND DO SOMETHING PRODUCTIVE, GODDAMNIT!