It's natural to move around. I totally agree with drifter, just don't turn your back on anyone.
I was speaking in front of middle school classes all day today. Since the topic (the election, how Congress works, etc) wasn't entirely engaging, I tried to up my energy level a bit to keep their attention, and that involved moving around as I spoke, stepping towards the kids who were asking questions, etc. I'm not the best public speaker, though I have to do some for work, but it was fun.
The three most common questions? "Should I be worried about Ebola?" and "Have you met President Obama?"
Those seventh graders, keeping their eye on the big picture.