Bukatsu, or club activities, are a staple feature of anime and manga set in Japanese schools.
In the Japanese school system, extracurricular activities are strongly encouraged, if not outright mandatory. Every place of learning, from grade school to university level, has a slew of clubs catering to the students' interests - Physics Club, Track-and-Field Club, Book Club, Photography Club, etc. Participating in these clubs takes up a significant amount of "free" time.
If one is not interested in participating (an attitude viewed with suspicion and even marginal hostility, as being anti-social), one can be an inactive ("ghost") member of an existing club, or one can join the ranks of the clubless outcasts, sometimes referred to as the "Go-Home-Early Club".