Depression

So, recently I've been feeling a bit down for the last 5 months, I'm bipolar.
My teachers are worried about me and one wants to speak to me, I trust him, (my head of year) I've never lied to him. I've always told him the truth about what I've done, (if I was in trouble or for what ever occasion.) 
I've gone back to old habits, of dealing with depression (self harm.) In year 8 (I'm now year 9) a teacher sent me to his office because she saw some of my cuts, so he knows that I Have done it before. (In year 7 I was scratching and I got sent to my old head of year.) 
Last time he helped, but I don't know how to explain it as I'm not ready to tell my parents. (Sorry for grammar) 
Need reply ASAP he wants to speak this week...