帰宅部 is a classic and well-known Japanese slang that literarily means ‘a go-home “club”. It is used to indicate that you don’t belong to any club at (junior) high school, while highlighting the fact that you are still a part of the group of people who do not join any club. Although its tone may sound like a joke or something, it is rather a neutral word that describes such status.
部活 (Bukatsu): School Club Activities in Japan
In Japan, most people join 部活 (bukatsu: a club) at junior high school and high school. There are roughly two types of clubs at school: 運動部 (うんどうぶ; sports clubs) and 文化部 (ぶんかぶ; arts/non-sports clubs). Popular sports clubs in Japan include baseball, soccer and basketball clubs, and those clubs are typically very serious and strict. For instance, usually you have mandatory practices 5 or 6 days a week after school, and some clubs further require or ‘recommend’ you do 朝練 (あされん, morning practice) before a class starts. The social hierarchy in those clubs, especially in strong teams, also tends to be rigid, and you must be very polite to your coach and senpai (older students) using keigo (honorific language). Sometimes, some clubs are penalised for the abuse of power found in the team, such as the coach’s violence on the students and sepai’s bullying on kohai (younger students). Some people argue that those extreme bukatsu cultures are anachronistic, and that they promote blind obedience to the leaders or authorities.
On the other hand, non-sports clubs tend to be more casual and relaxed, although some clubs especially 吹奏楽部 (すいそうがくぶ; brass band club) can be as strict as sports ones depending on a school.
帰宅部 consists of two parts 帰宅（きたく） meaning ‘go/come home’ and 部 (ぶ) meaning ‘club’. The first word 帰宅 is usually used as a suru-verb, ‘帰宅する’.
Because I belonged to 帰宅部 (‘a going-home club’) at high school, I would read a lot of manga after school.
Synonyms in English: not belong to any club at school
Related Words and Phrases: 帰宅する (go home), 部活 (club at school)