Your past self

Solid advice during my childhood was already given. I'd just echo it minus some parts:

You've got the right stuff
So make your mind up
Find the courage inside of you

If you're strong you'll survive
And you'll keep your dream alive
It's the Battle Frontier
Pokémon!

Be the best you can be
And find your destiny
It's the master plan
The power's in your hand
Pokémon!

I'd tell myself GO MAKE REAL FRIENDS NOW

" stop being lazy and actually pay attention in those lessons"
" Do not become friends with this girl who is a snake and will betray you"

I would say "Be brave. Be strong. Always keep a hope inside yourself. Very hard times wait for you in the future but in the end everything will be ok and you'll find your true happiness in life. And I always be with you despite all these hard times".

I'd tell myself to open up my mind more, you don't know nearly as much as you think you do. And to learn that relationships in highschool mean little and less, so don't care so much about it later on - your time will come.

Also quit being such a dramatic little bitch

Enjoy it whilst you can - it's only going to get worse from here on out. You think you have problems now? Just you wait.

...also, save your money.

"Please grow up faster and stop doing nothing with your life."

I would tell my 14 year old self: "You will become exactly who you want to be. Be prepared for that."

Because through bizarre, roundabout, and often tragic circumstances, it's actually technically kind of true. Which is really damn weird/satisfying to think about.

As for younger than that, "Be kinder to the people who deserve it."

To thirteen year old me:

"Just accept yourself already. Yeah I know almost none of your 'friends' will accept you after that, but you're going to be growing apart from them really soon anyway, and you'll feel much better about things regardless. So just do it now and save yourself from some of the later damage."