Very little hitting time per hour. Too much waiting for a turn can make tennis seem boring. Lack of individual attention can leave some students behind. Kids taking other lessons who want an additional chance to play. Families who can't afford more expensive instruction.
Group lessons are probably the most common format in which kids learn tennis.
The structure of group lessons ranges from groups of three or four taught by an experienced pro to groups as large as ten taught by a local high-school player. The amount of hitting time and instruction is inversely proportional to the group size, so smaller is almost always better. Generally speaking, kids will learn faster and more soundly with an experienced and credentialed pro, but younger kids sometimes feel that a high-school age instructor is more fun. In general, kids learn gradually and have a lot of fun in group lessons. Comfortable number of companions.