It's late and I'm just off to bed, so hope this makes sense. But just my tired thoughts on options available to you
The Police will/should have a child protection team, Police Officers who deal with this on a daily basis, are more experienced in dealing with disclosures, CP issues etc. They will share any info with Social Services, so it might be worth you ringing them and sharing your concerns about the situation. (not sure of their "working hours" ie if they work weekends ) They also have the power to remove a child to a place of safety, if she's not safe at home.
Social Services should also have an out of hours team that you could contact - local Police should have a telephone number for them too.....Not sure if you can get in touch with them by ringing the local area office.
If you are really concerned for her safety you could also ring NSPCC who might be able to point you in the right direction as to how to contact local team. They will also refer on to local SS ie fax any information/concerns they receive. Child line might be another option for advice and information.
If you think she's safe this weekend and you and your friend ring Social Services and make a referral on Monday, knowing the child's name, ie first name, surname, rough age, and school should be able to pinpoint which child, and get the rest of the information Child Protection Team need, plus any concerns/information school may or may not have.
Hope it works out ok Op