Get access to the room ahead of time. Practice in the empty room. Get comfortable there. Use the space. You'll notice how teachers and presenters don't stand rooted to a position? Why should you? Assuming it's a classroom, and not a hall, move around the classroom. If there's a media element, you'll need a remote, or an assistant. This will both free you up to move, and also allow you to flow better if you don't have to worry about mousing around the screen. OWN THE ROOM.