Lea, by your description, you're a lot like me when I was your age. I wasn't failing any classes that drastically, and I didn't cut myself, but I slacked off all the time (hey, I still do sometimes, and I'm a freshman in college now) and never wanted to do anything but internet. It seems really hopeless, doesn't it?
Well guess what.
It isn't.
To be honest, I really would rather talk to you in person right now. You can't have any idea how passionate I feel about helping you right now by simply reading this post. I'm not going to try to contact you, 'cause that would be weird, but understand that YOU CAN GET OUT OF THIS.
And you know what? You've already started. You just admitted you have a problem. Not that I want to equate this to AA, but in any bad situation, admitting that you have a problem is the first step.
Before anything, if you haven't admitted your problem to people you know, especially adults you trust (read: your parents, but if you need support when you have to admit it to them, and trust me, you will, ask another adult you trust for support. Note: you NEED to tell them too so they support you. That's why you have that bond of trust). Verbose, BSOD, and mostly everyone have said it, but you should really see a psychologist. I don't mean any offense, but you may have a mood disorder and to see a psychologist or psychiatrist, you're going to need to get them to consent.
Now for the positive: A lot of people have raised good points. I'm glad I refreshed and read BSOD's, cause he nailed a lot of good things. You are growing up and getting more responsibilities. The first thing you need to do is accept that, and take them. Trust me, it'll be hard, and you'll do it more and more as you get older, but you get used to them. It's still hard for me to take new responsibilities; it's hard for pretty much everyone. I'll give you an example: I spent all of my bank account on STUPID stuff over the weekend through Tuesday (no regrets on Assassin's Creed though), and I don't get paid till next Friday. Pretty much all I've got until then is the stuff I already own, a room, and every meal except Sunday dinner. But I've learned a lesson from it. That's what you do when you take those responsibilities. You learn from them. And you know those classes you're failing? If you start to take the responsibilities in those classes, you just may learn to like them. I know I did.
Yes, I cut myself…
Lea. You need to stop. It's not helping and it never will. Just by your OP, I can tell you're ashamed by it, and right in the sentence before:
I’m the only one you’ll see with cuts on my arm…
Do you know what you're suggesting? You're suggesting you try to find other cutters at your school. You're self-conscious about it, and you need a sense of belonging. It's not dulling the pain, it's only making it worse. You need to find healthy and uplifting activities to replace that. You know how you said you like to draw, and how you said it's useless? I don't think it is. You already enjoy it. Now I'm no psych major, but I think if you try to draw things you like, things that support you, that's already a start. Draw your fears and worries being destroyed by the ways you want to change! You need to admit it's not impossible to escape what you're feeling, and truly believe it. Show yourself achieving it. I can't draw, so that's not a strength, but I have a great imagination and mind, and I'm really good at putting a cause to a future effect. My strategy is that every time I finish an assignment or manage to control myself monetarily, I imagine that it's one more step towards success in the future. Use your strengths to cheer yourself on, and replace those negative habits with your positive ones.
Realize, however, that you need to reward yourself for doing those difficult things. Use the internet. Draw yourself a prize. Sing a victory song (recommending the FF6 mix of the Victory Fanfare, that's still an achievement song for me). But don't fall back into your old habits doing it. Remember how I said to get online? You need to set yourself a time limit and hold to it. Have your parents hold you accountable if you need it. When my parents MADE me get off the Xbox to do homework/read instead/do other stuff, I soon found it easier to concentrate on homework because I knew I couldn't use it anymore, and using it after my homework helped even more, as I could look forward to it. And I know you can lie. I did. Make sure that your parents check that you did it. Write the whole assignment in a planner so they can double-check that you didn't half-do it.
A final side note: I've told you to give several responsibilities to your parents. Now look, I know they're annoying at that age. I was thirteen once, too. You need to realize that if you trust them and ask them for help in the process, they'll be happy that you still trust them and can give your problems to you. Preteen and early teen years are difficult for parents; a lot of kids that age have trust issues with them, and it really puts them down. Show them you can be a responsible young lady.
That's what I truly believe you can be with some effort, Lea. You may not think so right now, but I sincerely think that underneath all of that turmoil is a wonderful young woman who can be successful in whatever she does. That sounds kind of creepy, but please, stay just a bit longer. I want you to pull out out of it, and I know you can. I did, from similar circumstances, and I thought I was a wreck. You just need to try, and use some tools to help you.
I wish you the best.
Somehow I managed not to get ninja'd once. That's crazy, I wrote that for an hour.
EDIT: Twins two down reminded me of something I was going to write anyways – you're obviously a smart girl. You talk to people five, ten years older than you, and you do so at a level that astonishes me for a thirteen-year-old. You're not stupid. Just put that great brain of yours towards work instead of towards avoiding it.