Am I responsible for the school fees of a child I look after but is not mine?

The whole question

My daughter’s friend has been living with us for the last few years. 

With her mother's permission I took the child in as she was desperate and wanted to get her matric.  

I completed all the school forms but the person responsible for the school fees is her father, whose details I listed.  

Every year when the school sent the forms home to be updated I listed her father’s details for the responsible person to pay the account. Yet every time the school sent statements, it was made out in my husband’s name. We phoned the school and gave them the correct details.

The girl's report was withheld due to outstanding school fees. Again I gave her father’s details and I did the same when the school governing body withheld her report and insisted I sign an acknowledgment, which I never did.  

I received a call from the school's lawyers saying I’m responsible for the school fees. I explained the whole story to them and said that the school has her father’s details as messages were sent to him and he made contact with the school and also had a meeting with the principal acknowledging the debt. Arrangements were made but I have no information on those what arrangements.  

Where do I stand?