I'm sorry that you're going through this. We went through a similar situation with my niece.
We all tried to get help for her via SS, it was like talking to a brick wall. Her dad made calls to SS, asking, begging for help. They didn't want to know. Just as in your situation, the child was old enough to decide where she wanted to live so she was allowed to stay with her mom. I wouldn't say she lived in squalor, but she wasn't being looked after. The front door was like a revolving door with mom's many boyfriends. There was weed in the house. She wasn't encouraged to wash regularly, didn't have many clothes. Anything we provided; money for birthdays, dinner money, pocket money for example, all 'vanished'.
It is incredibly frustrating when all the people that SHOULD help WON'T help.
Is it any wonder why a child comes to serious harm and then there's a serious case review and 'lessons will be learned', yet when they have the opportunity to intervene, they often don't.
Parental responsibility - if they weren't married, I think your son would have to apply to the court for this. School then MUST treat him as an equal in terms of his rights as a parent. He needs to get this ASAP.
Finally, I am so glad you are continuing to fight for your granddaughter. She needs you. My mom was exactly the same x