Idol School (Korean: 아이돌 학교) was a 2017 reality girl group survival show on Mnet. The program aimed to form a 9-member girl group from a pool of 41 female participants, training them in singing, dancing and other "subjects" over an 11 week time period.While sharing similarities to another Mnet survival program, Produce 101, the two shows differed in many ways including its number of participants, voting system, and number of winners that will form the final group. Another of the shows greatest differences was how they treated the eliminated contestants. In Idol School, participants were given the chance to continue studying in the 'normal class' where they would continue studying in a different class off-campus (off location) under less public scrutiny, for a chance to debut in a future group.The show premiered on July 13, 2017, with the final episode airing on September 29, 2017. The new group name was revealed to be Fromis 9, meaning "From Idol School", but pronounced similar to "Promise" in Korean, and the final line means the never-ending possibilities and the promising and prosperous road ahead after graduation that would last forever. The new group members trained and released albums through Pledis Entertainment, while their group activities were managed by Stone Music Entertainment/Off the Record Entertainment, as a joint venture with CJ E&M.

In August 2021, after a meeting about the group's future, Fromis 9 transferred management to Pledis Entertainment.