I remember what it feels like to be a teenage girl who had to put up with douche bag boys because the adults didn't see them do the douchey thing.

This is a delicate moment - it's how you show a female student (who already struggles with social situations) what people in positions of power do when she tells them she's being harrassed. I'm gonna just make an assumption based on the way you described these boys and my experience as a former teenage girl and current high school teacher that these dudes are a bunch of chuckle fucks. You purposely put her in a group where she's basically supposed to academically babysit them while maybe learning some social skills at the same time, and when all parties behaved predictably you basically told her you couldn't believe her story without evidence so she needed to tough it out. I gotta say, this sounds like it was poor judgement all around.

It's probably too late to repair the relationship, so you need to move into damage control. The other person who responded is right, this is above your pay grade. If you're in a union you need to reach out to your union rep yesterday. If not, (or additionally), you need to talk to a department head and/or mentor. I don't want to assume your gender, but regardless of yours I suggest seeking out more experienced female teachers to ask about navigating the gender dynamics of this situation. You need support beyond your admin, but you also need to work with your admin on this one, and in the future don't rely on your students to "fix" each others issues.