It's alright. She needs you, she's still a child. This will live with her for the rest of her life. You need to trust her, believe that she's telling you tht truth because if she thinks that for one minute that you don't, it could cause permanent damage to your relationship with her. If on the other hand, you go and think about it, write it all down if this helps, then you tell her tomorrow that you believe in her, that you are always there for her no matter what and that you don't think that she should drop the charges at the moment, she'll remember this too and you'll have a closer relationship.
I do strongly suggest you get some expert help with this. Others have suggested the NSPCC, I know that Bernardo's also help. Call the school nurse on Monday and tell her about the reasons why your daughter wishes to drop the charges. Your daughter has a high level of trust in the school nurse or she would never have gone to her. It sounds like your daughter is more afraid of what's oing to happen to her step father rather then herself. Abusers have alot of power over their victims, the abuser's the one in control, the victim takes it back when they complain. As you've read, sometimes it takes a very long time for this. She's gone through so much already, examinations, meetings, interviews. She has a deep love for him and she feels the need to protect him because this is how they work, this is how they make sure the child they are abusing keeps quiet. No one would willingly want this. Look deep inside and ask yourself if she's right. She's so very brave, you must be so proud, you need to show her this.