Not sure about the accuracy since I started watching from 4th generation era. Credits to 2channelers.
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Rumi was graduated from idoling!!! and entertainment biz for health problem, but rumor said she was forced to graduate after scandal. But she was indeed had health problem(she looked like Suu-chan's condition before graduation). No special graduation event, other than sudden announcement at 4th live and a special episode compilation "Zumidoling!!!" episode #469.
Sayanee, Etochan, Miraccho, Michelle graduation announced properly with special graduation episode 486-489 and graduation live 5th live. They even had a 30 seconds graduation CM-like video on the opening of some episodes.
Maia was resigned from idoling!!! and entertainment biz after just 2 months since joining idoling!!! (culture shock?). Only in the show for 2 weeks. Too busy? Since she was not graduated "properly", she is now the missing number. She has come back to entertainment biz and is now a member of idol group Chu-Z.
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CMIIW veteran idolingers