Stress
Keeping a close eye on one kid...not so tough. Try ten or fifteen at once. (Source)
Teaching dozens of kids how to swim can be kind of hectic. You are responsible for all of them. Some don't know how to swim and could drown if you take your eyes off of them. It's a little less stressful giving a one-on-one lesson, but even so, some kids just live to torment their teachers.