Swapping horses, definitely. For my first several years of riding when I didn't own my own horse I was on a different horse almost every week, even as a raw beginner. If you have a set of good, capable lesson horses I don't see a problem with switching even the beginners around weekly. I think the only reason I'm a capable, good rider is because I rode about 15 different horses in my first two years of riding, not only that but they were all different sizes and breeds, so I learned that I'm capable of handling any horse, no matter the size. Granted, if a rider is very nervy and very, very unsure, I can understand keeping them on one horse, but I still feel that swapping out is the best option even for beginners. In my opinion, it helps build confidence moreso than keeping them on one horse.
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."