For girls, depression can be a result of hormonal changes in the body. I got depressed about age 12, and it continued till I was 17 or 18. I began to realize it corresponded to monthly cycles, and did some practical things to help. Even though I was not a Christian, I know God was helping me find these ways of coping.
So, you just may outgrow it!
But if not, you might want to talk to a school counselor. They can send you on to a proper doctor if it really is depression. My advice for now is to read the Psalms. They helped me when I became very depressed due to severe Rheumatoid Arthritis and not being able to do anything, and being in so much physical pain.
God is there for you, ask him to help you.
"Heavenly Father, I pray you would help the OP to get some help for her current depression, and that you would also show her your love! Let the pages of the Bible become alive for her, and give her the hope that knowing you brings. In Jesus name I pray, Amen."
This is the verse God gave me during this period of time, which is also the same verse posted above, but in a different translation.
"5 Why am I so depressed?
Why this turmoil within me?
Put your hope in God, for I will still praise Him,
my Savior and my God." Psalm 42:5