What type of turns are you doing? In Junior High, many years ago, we did both pinwheel and square corner turns. For both the pinwheel if you have them watch the people around them and keep the line straight it should be no problem as long as they have the picture in their heads. For square turns it was always a matter of knowing that you are just marching next to a new person until it was time to turn and than you are lined back up with your old partner. I think the key is to have your best marchers on the outside of the formation to be a guide.
Hope this helps it has been a while since I parade marched.