The official Twitter account for Love Live! School Idol Festival ALL STARS has announced that the app page for the game is up on Google Play and the App Store and you can pre-order/pre-register for the game now. The game is set to release on September 30 but the staff have made it clear that the date is tentative and may change. The official release date will be announced at a later time.
You can check out the links below:
School Idol Festival ALL STARS is a successor to Love Live! School Idol Festival which released in April 2013 and June 2013 for iOS and Android respectively in Japan. The English version of the game launched in May 2014.
School Idol Festival ALL STARS was announced at Tokyo Game Show 2017. It will feature all 3 school idol groups (μ’s, Aqours, Nijigasaki High School Idol Club) from the 3 Love Live! multimedia projects: School Idol Project, Sunshine!!, and PERFECT Dream Project. Unlike the Love Live! School Idol Project and Sunshine!! TV anime, both μ’s and Aqours will be active at the same time, so the game’s timeline is separate from the canon set in the anime.
Love Live! School Idol Festival ALL STARS is tentatively set to launch on September 30 for Android and iOS devices. There is no official worldwide release announcement at this time.