Club activities. Groups formed by multiple students with common interests to conduct hobby-related activities during their spare time.
In anime and manga, it's often a small club with only 5 to 7 members, but in reality, it's more common to encounter larger organizations with a dozen or more members, often named "XX Club."
Gongsun Ce hadn't joined any clubs in high school; he only heard about their dull aspects from friends—ranging from virtually no club activities, most participants being never-seen ghost members, to interactions often centered around a small clique, making it more of a social scene than a hobby pursuit, and so on.