If you tell your teacher you cut yourself, Is it legal in the UK for them to call/tell my Mum?

[I choose not to display my name ;)]

Okay. I need to explain before I get a load of hate telling me I shouldnt.
So, I'm nearly 13 Years old, And, I cut myself. Not very badly, but still a tiny bit painful.
Yes I have help, Someone who's been in the same situation as me, but worse, is helping me, and she recommends that I see a teacher when we go back to school next week.
Last year (Year 7) I won Netballer of the year at school, I received a Trophy, Jumper ;D and certificate. Even though that was last month, I'm still very proud of myself.
One teacher at school, Teaches me in Netball, and she only joined around May/June time. But she is lovely. She is now my Co-Mentor for year 8!
I was thinking of letting her know, So she could help me, and so my friend doesnt have to do this alone. My friend is going to University now, So i wont see her ;( But i can stay in contact over the phone, Facebook and Twitter, etc etc. But, Im so afraid of telling my Teacher incase she calls home or tells my Mum at a parents evening or something. I thought it was confidential but some still say that she can? Help Please?
Sorry, What if i ask her not to call home?