Tips for teaching in an Urban classroom?

I’ve finally landed my first teaching position. It will finish out the school year. My first concern is that I have 8th graders. With 8th graders, they have had spring break and already registered for high school. My thought is Middle school will be a joke to them, at this point. Second, it is in Memphis. I am not from the country, but my town isn’t quite as big as Memphis. I know this atmosphere will be something new for me, and I will have challenges.
Does anyone, preferably an educator or parent, have any tips on making sure the kids know that I mean business from the start? This is a tough situation, considering the point in the year that I am coming to them, but I am an optimist. So, I figure, if I get a few tips and use some trial and error, I’ll make it through the 2 months at least. One plus it that the administrators seem to take no prisoners, so I’ll have good back up.
I am a nice and positive person, but I’ve been told to put that away for a bit, to make sure they don’t take me as a joke. I truly understand that, but how soon can I come out of my shell? I learned a lot in school, through observations, and students teaching, but it is a bit different when you get your own class and you’re on your own.