kids often pick up things about sex earlier than you would think and you don't always know why. To be on the safe side, I would stop having the two play together. Do not put the kids in the middle of an argument. I would get all the details from your daughter and see if her stories are consistant, if they are, I'd say she's telling the truth unless you know she has lied a lot before. From what you say about the boy, he's only admitting some of what happened. If he didn't do it, he would have denied doing anything. Whatever the truth is, I'd be very suspicious. Try asking the parents to get a play by play of what happened. I would also contact the police and see what they have to say. If this boy is acting out sexually, he could have accidentally seen a mature video or even have been molested himself. I'm sorry to hear about your situation, but I feel there is a need to really find out the truth even if a higher power steps in. Also, continue to educate your daughter the difference between a good touch and bad touch and what to do in a such situation. Praise her for telling you what happened.
You say you don't want to make a big deal out of this until after school ends, but the fact is this IS a big deal. You need to get things done while all of this is fresh in your heads. By failing to act right away tells your daughter that whatever happened to her was not important enough to deal with right away. As a mother, it's your duty to protect her at any cost. You, yourself, were molested and had to live with years of guilt. Don't delay getting your daughter help because one day is one day too many.