Meet the students on their turf. Go to their school. Sit on the floor. The school environment worked well for us, because it was familiar to the participants but was structured enough that the youth didn't view the focus group as recess. Don't dress up too much.
Icebreakers are optional.
We didn't use them. The students probably wondered what this focus group business was all about, so we got right to the questions and ended the suspense. Explain that the focus group session is not a quiz. Explain that there are no right or wrong answers and that you just want to know what they think. They may not believe you at first, but you can drive this point home by being nonjudgmental throughout the session.