Get Adderall.
Quit being lazy and get a hair cut.
Save yourself a ton of trouble and DO YOUR HOMEWORK.
Don't get caught up in WHAT to do/say/write and just do SOMETHING (in schoolwork and elsewhere).
Don't stop taking ninjutsu.
Yes, I know you're a ninja, but quit being so gorram invisible. You're a gorram titan amongst men in both ability and stature. Stand up straight, be confident in the fact that you're exactly as badass as you think you are and that people will recognize that, but be humble about it.
These girls will pretty much be the most important people in the world to you. Don't let any of them friendzone you.
*You're wrong, B. doesn't have a date to prom. I know you're in love with C, and and have no urge to go unless it's with her, but ask B out. If you don't, you'll hate yourself for not doing this for her. Keep track of her and always try to be there for her - she needs a friend and guidance or else the world will try and destroy her. She's one of your best friends, she's the girl you liked before you liked girls, and in a few years she'll be the hottest woman you've ever met. She doesn't have a date because people think she's out of their league. She's not out of yours. Trust me.
*C is the strongest person you know. Your equal. Never underestimate the shit she's been through. Be confident and direct with her. Be the best, nobleist, most honorable, caring, heroic person you can be. It's the only way you'll actually be worthy of her. DO NOT GET FRIENDZONED. DO NOT WAIT. If you don't love this girl, you're an idiot. Give her space unless you want her to clam up and lock you out. I've almost known her for a decade. Totally worth every drop of time you can spare.
*R is just like you. Nearly your equal. If you and C aren't dating for some reason, ask her out and just be completely confident that you two have basically everything in common even though you don't know her right now. If you don't date her, warn her away from Alex and warn her about Stephen. If she does "date" Stephen, straight up and with great gravitas tell him to quit being a dick, treat the woman right, quit stringing her along and actually really date her.
Be her friend, not his.
*Don't speed, it costs you money. Seriously.
This is just on the short list of things I'd tell my teenage self.
Or, those are the things I'd tell myself if I was going to tell myself anything.