It is possible that your grandchild has been sexually assaulted at school, either by a teacher of a pupil, and threatened into silence about it? Her behaviour would be a fairly typical reaction to such a traumatic event, and if it's the case she will need specialist counselling.
btw, your Social Services department's response is so far from acceptable (you wrote " Another time she phoned social services after she had been physically and verbally abused by my grandaughter that she was at the end of her tether and told ss that she was going to kill her daughter and herself, ss said well theres nothing I can do now as it's the weekend") that I strongly advise you to make a formal written complaint to the Director of Social Work in the relevant local authority - Citizens Advice could help you with that if you feel unequal to the task on your own.
Every Social Work department must have an out-of-hours standby service to cover situations just like this, and any social worker who reacted as you state, needs to be instantly dismissed.
I'm a retired SW myself, and I can't for the life of me understand why your Granddaughter has not been admitted to a Children's Unit in a psychiatric setting when things are as bad as you've described. She, and indeed the whole family including yourself, desperately need support. Please write to your MP as well as the Director. I found, when I was working, that an MP's interaction got a lot of attention.
I wish you the very best of luck.