I could only find it in me to tell one friend, the friend I trust the most, even though she may not be the one I get on with most. I know she won't tell because she knows that when I'm ready I will stop. She understands, I explained it and she listened. On many occasions I have tried to tell my mum I cut, but each time I froze and couldn't do it, she's seen them but I lied, that was perfect timing but I couldn't do it, but trust me, when someone knows it's so much nicer, I felt pressure be released and it means when I'm at school I can escape the awkwardness. But she is your best friend. She will understand and be there to support you, she will care about you and therefore do what she can to support you and if you don't want her to tell, she won't.
PM me if you want to talk. <3