Congratulations on reaching six months SH-Free!

I would say maybe approach your mum. Just start talking to her, don't ask her questions relating to what she found out today. She can only stay quiet and ignore you for so long. I know you probably don't want to take that first step to initiate a conversation, but it might be for the best.

Schools are supposed to inform your parents if you self-inflict an injury upon yourself, they're required to by law. They believe you are a danger to yourself (not necessarily others, but it has happened in the past) and need to get it resolved. They might also recommend you talk to a counselor, and there's nothing wrong with that.

I know you did go half a year without SH, but tell them about it if you do get assigned a counselor. Explain what caused you to relapse and maybe they can discuss that with you and not the fact you SH.