Hi, there.  I'm sorry you're feeling so isolated right now.  That is one of the symptoms of depression, but there are others, as well.  Do you have anyone, like a school counselor, that you could talk to about this?  Have you had a physical exam by a doctor in the last year?  Sometimes there are medical conditions that can cause depression and you'd want to rule that out.  It sounds like you may have a little anger going on, too.  Unacknowledged anger can also cause depression.  Are you able to talk to your parents at all?  Maybe you could tell us a little more about what's going on in your life.  The teen years are not always the easiest for anyone, so it can be hard to tell sometimes if you're going through a normal mood fluctuation or if it is depression.  If it continues for several weeks or more, it might be worth having an evaluation by a psychologist or counselor of some kind.

I hope this was somewhat helpful, but feel free to write more if you want - there's usually someone around here checking in.  Take care.