=== Før-debut: Dannelse gennem ''Idol School'' og "Glass Shoes" ===
I marts 2017 blev det annonceret, at Mnet, den samme kanal der var ansvarlig for adskillige bemærkelsesværdige ''survival shows'' inkl. ''Sixteen'' og ''Produce 101'', ville lancere et nyt ''reality survival show'' med titlen ''Idol School,'' for at danne en ny pigeregruppe. <ref>{{Kilde nyheder|url=http://entertain.naver.com/read?oid=311&aid=0000716543|title=Mnet 걸그룹 육성 예능 '아이돌학교' 론칭…"졸업 즉시 데뷔" (공식)|last=Lee|first=Ah-young|access-date=30. september 2017|publisher=Xports News|language=ko|archive-date=30. september 2017|archive-url=https://web.archive.org/web/20170930132239/http://entertain.naver.com/read?oid=311&aid=0000716543|url-status=live}}</ref> Showet havde premiere den 13. juli og sluttede den 29. september 2017 med Roh Ji-sun, Song Ha-young, Lee Sae-rom, Lee Chae-young, Lee Na-gyung, Park Ji-won, Lee Seo-yeon, Baek Ji-heon og Jang Gyu-ri som de ni medlemmer af ''fromis_9''. Det endelige line-up blev udelukkende besluttet gennem live- og onlinestemmer fra seerne. <ref>{{Kilde nyheder|url=http://www.koreaherald.com/view.php?ud=20170712000706|title=Wannabe K-pop stars enroll in 'Idol School'|last=Yoon|first=Min-sik|access-date=30. september 2017|publisher=Herald Corporation|archive-date= 4. oktober 2017|archive-url=https://web.archive.org/web/20171004102144/http://www.koreaherald.com/view.php?ud=20170712000706|url-status=live}}</ref> <ref>{{Kilde nyheder|url=http://entertain.naver.com/read?oid=117&aid=0002932712|title='아이돌학교' PD "'프듀101'과 차이점? 연습생 아닌 일반인"|last=Lee|first=Seung-gil|access-date=30. september 2017|publisher=My Daily|language=ko|archive-date=30. september 2017|archive-url=https://web.archive.org/web/20170930174953/http://entertain.naver.com/read?oid=117&aid=0002932712|url-status=live}}</ref> <ref name="TV Report">{{Kilde nyheder|url=http://entertain.naver.com/read?oid=213&aid=0000990621|last=Park|first=Gwi-im|access-date=30. september 2017|publisher=TV Report|language=ko|script-title=ko:'아이돌학교' 데뷔조 1등, 걸그룹 센터→리얼리티까지 '파격 혜택' [공식]|archive-date=30. september 2017|archive-url=https://web.archive.org/web/20170930131810/http://entertain.naver.com/read?oid=213&aid=0000990621|url-status=live}}</ref> Pledis Entertainment, ledet af CEO Han Sung-soo, administrerede gruppens træning og debut. <ref>{{Kilde nyheder|url=http://entertain.naver.com/read?oid=213&aid=0000990830|title=뉴이스트X세븐틴 대표, '아이돌학교' 데뷔조 9명 프로듀싱 확정 [공식]|last=Park|first=Gwi-im|access-date=30. september 2017|publisher=TV Report|language=ko|archive-date=30. september 2017|archive-url=https://web.archive.org/web/20170930174956/http://entertain.naver.com/read?oid=213&aid=0000990830|url-status=live}}</ref>
Musik