There's an old saying, "Where two or three are gathered, someone will spill something." Okay, it's really Jesus who says, "Where two or three are gathered in my name, there am I with them (Matthew 18:20)."
Just a general principle to throw out there in youth ministry: If one or two show up...do the program or event! You have a divine appointment with each person who shows up, so make the best of it.
To be honest, I was a bit nervous about a recent theme park outing when one young man came and five young ladies--four of which were middle schoolers, two I'd never met. I began to think about what in the world I can offer middle school girls? And before long, God started showing me some pretty cool stuff. I got a chance to see into my own future--about 10 years down the road when my own daughter will be their age and doing stuff with friends. I just smiled.
So what do you do as a male youth leader when working with middle school girls? Well, I hope it goes without saying that we need to be smart about it and there are obvious don'ts. (If you do not realize this as a male youth leader, get out of youth ministry now and get help, please. I mean this with as much love and grace as I can muster when there are a lot of people out there who hurt kids.) Other adults should be present. If this isn't possible, be sure to meet in public places. Never travel alone with just one youth in a car (boys or girls). I look at it like I'm their older brother, but often girls will need the older sister, cool aunt, or sweet grandma figures in their lives more than they need you. Be okay with that, guys.
Back to the question though: what DO you do? Love them. Laugh with them. Protect them. Let them dunk you or knock you off your tube in the lazy river. Let them treat you like they'd treat an older brother. Remind them that boys have cooties and are gross and to stay away from them. (Dad's like that a lot!)
And remember, most of all, middle schoolers are awesome! Even the guys.