I echo what lepry has said - this week could be a chance to stop self-harming.
If you can explain to one of the leaders they may be able to support you. If you could download some material - e.g. from
www.scar-tissue.net or the Secret Shame site, which are reputable sites giving information to friends and carers of those who self-harm, it might help you to explain.
People who have never done it find it hard to understand how it can serve a temporary purpose (much as getting drunk does for others) but, as with using drink, the effect is only temporary and there are adverse long-term effects and addiction problems, so we need to learn other was of handling the stress or other feelings which make us self-harm.