Thanks everyone for the input. I ended up focusing less on the balance issues, and instead on what I think was the root cause. The students that were skiing in the back seat were doing so mainly when they were using a braking wedge to control their speed. They were perfectly capable of keeping their CoM over their feet on easier terrain, and in a lot of varied conditions. So it wasn't their balance that was the problem, it was that they were relying on braking movements to control speed. I spent a chunk of the morning working on turning up the hill and using turn shape and line choice to control speed. After a few runs, I saw a big difference with most of them. For most of them, this wasn't new. They had done it before, but just stopped doing it unless they were reminded.
Unfortunately, the student I felt needed the most help didn't show up in the morning when we were really working on things. She did show up in the afternoon, but I had promised the group that if we got through all the "work" I wanted to do in the morning we could do a lot of fun stuff in the afternoon.
Overall, I think it was a success. A few of the students learned (or maybe re-learned) how to control their speed without a braking wedge, and it created a more organized situation for me. There are a few wanderers in the group that are always off on the side trails, so by having them ski in a line and follow my tracks helped to wrangle them in.
That brings me to another question I have. With regard to the wanderers, I'm struggling with where to draw the line on letting them have fun, and keeping order in my group. While I want them to have fun and explore, I have a huge obligation to keep them safe and to be aware of there whereabouts at all times. So I'm balancing the idea of a controlled organized group and allowing them to do whatever they consider to be fun. With the limited amount of snow we've had in the northeast this season, some of the side trails are marginal at best. Small rock ledges that are usually covered in snow are now exposed, cover is thin, and things can get icy. This doesn't stop 8 year olds from wanting to ski there. It also puts me in the position where I have to tell them they can't do something they have done many times before. For a few of them, there are no limits to where they will ski. If there is a half inch of light powder covering bare ground on the side of a trail, they will try to ski it. And after that doesn't work out so well, when they see another patch just like it 20 feet down the trail, they will try it again. For these few students, fun stuff or free skiing means off the trails. So how can I allow them to have fun, but keep them from doing stupid things that probably aren't going to end well.
The other issue with the wanderers is that they hold up the rest of the group. When they decide to take a "side trail", they usually don't make it up the mound of snow they are trying to get over and have to hike, or they fall and lose a ski. The rest of the group skis ahead and stops at the next checkpoint, and waits...and waits...and waits while they repeat the process of trying to ski up, off, and over every snow pile, rock ledge, and bump on the mountain, falling down, and losing a ski. The rest of the group will free ski the trail. They might take a few of the more accessible side trails, but they spend most of their free runs actually skiing. So how can I reign in the wanderers, but still let them have fun? Their idea of fun seems to be strictly limited to runs with a lot of side trails.