My friends got mad at me for not attending a friends bday

I'm in a gc now. Its a okay gc but there's this one girl who is going out for her bday. We were already planning it two week ago but something came up. I told her the day before (today) after school and she got mad at me bc apparently it was 'very annoying' she said 'I could have told earlier and I'm saying it last min'. I had only just found out that I was busy tmr and I explained to her that something important came up. She kinda went on about how I was saying bs and she got one of her friend 'ann' and they started being rlly ganging up on me. I just stopped talking in the gc and they continued saying bs. When one of friends 'a' spoke up Ann got mad and said 'no one cares' so 'a' was pissed and kicked her out the gc. The friend who bday it was tmr said that Ann did nothing wrong and I should have been kicked out and she called it favouritism. I posted the problem on my status for my primary skl friends to see but I forgot to take out a few ppl so it got leaked and they girl had another gc and talked shit about me and made rude photo of me.
Things got rlly bad so I had blocked her and I'm going to tell the school. BUT idk if I should tell the skl.
What should I do?