I have cut before, but not as badly as you are talking about. Instinctively I want to say dont do it, it its bad for you, yada yada yada. And it is, but that wont help why you do it. What I try to do when I fell like im about to self harm is distract myself from my current situation just long enough to calm down. Try reading, drawing, go for a walk outside. (if you live in the city, that may not be safe) Go lay in the back yard. It sounds kinda lame or stupid, but it may help. Also talk to an adult when you are ready, id hate for you to loose any function in your arm, leg, wherever you cut.
I don't blame you for not having the ambition to do the work, when you are depressed you don't want to do anything. I understand the feeling. Do you have one close friend to talk to. They may not agree or understand you, but it may be good to at least talk to someone in person. I hate doint it, but it helps to just breakdown to someone, they usually don't know what to say, but human compassion will make them want to help. If they are mean to you, then move one, they probably have problems that prevent them from being able to care about anyone.
Im going to P.M you my email, that way we can talk and hopefully help each other through things.