What's an ISS? Is it like something you have to fill out and have your parents sign? Anyway, I think I know what you are talking about...
You should NOT be held accountable for your friends' actions! Have you explained your story to your mom?
Try writing a letter to your principal (or counsler, or whoever you got in trouble with). Explain very CALMLY and MATURELY what happend, saying that although you got in trouble, you attempted to be the voice of reason among your friends, but you didn't have any effect on them. Perhaps try and apologize on their behalf.
Also, a quick question- did your friends rat you out!?
I am oh so sorry this has happend to you! I really wish the best of luck to you! I'm also sure your mom will not stay mad long. In fact, I think both you and your friends will move on soon.