I'm so glad you found us. Welcome. Is there a teacher or counselor or school nurse you can talk to? I see that you have tried to get help but have not been able to yet. I hate to say it but I think you need to just keep trying and trying until you get their attention. I know it's intimidating and hard but I'm afraid that you'll be overlooked otherwise. Do you have any family other than your mom and sister? Look at the top of this forum for a thread called Depression Resources. There are a lot of helpline number that you can call.
In the meantime you have us. There are so many different forums to post on and so many social clubs to belong to. You need not feel so alone.
I hope you feel better soon.