They do drills that would be more exhausting for a smaller class. Players in this group can still enjoy working on and learning fundamentals, such as control, consistency, etc. This size group is a great place to learn strategy. Every level of player. With a little help, you will improve your skill to become an intermediate player.
Intermediate players have their basics, but lack control, consistency, shot selection, power, etc.
Advance Players might have all the basics but lack just one or two shots to make them more of a complete player. For example, they might need more topspin on their ground strokes or more power in their serve. In general, a private lesson is a good tool for any level player. However, beware of the group lesson. Basic tennis skills and knowledge are left out in order to make sure so many people feel like they are getting a workout.