God Sharon, you poor, poor thing. It really does sound like she's got the upper hand and is out of control.
I'm so sorry.. This is awful...
I think you need and deserve some support... Not just from here but on a professional level too. A councillor arranged by School sounds like an important step in the right direction..
I wonder if it's worth a phone call to your GP too? If she's spent the night alone with a sixteen year old boy, there's a possibility she might be in danger of bring persuaded into having sex with him - clearly she's way to young to be making decisions about this sort of stuff. Also, perhaps the GP can refer you for some counselling of your own, just someone to support you and hold your hand through this difficult time.
Is there anyone in the wings that could be like a second Mum to her since she's locking antlers with you? A lovely Aunt/ neighbour/ cousin/ family friend? Sometimes a member of your extended " framily" can be just the sort of positive mentor/ role model a child like this needs? Even though they would be givingbthe same messages as you, sometimes difficult teenagers can't hear it from their own Mum.
Whatever happens though, you must take care of yourself and always stress to her that whilst you do not like/ want/ approve of her awful behavior, you do love her very much and always will do.
God, no-one said it would be easy, did they?... Babies and toddlers are a walk in the park in comparison, aren't they?...
Lastly - a funny little tip from me - when you are pushed to the end of your limit and then pushed another twenty percent and you feel on the verge of flipping out and losing control - imagine there is a fly on the wall documentary team with cameras all over your house and try to breathe deeply and behave in a way that you would be ok with if they were to show it on telly - with dignity and respect for both yourself and her.
I'm so sorry - I really haven't got any magic answers Sharon but you clearly need and deserve some support with this.