Hello! Thank you for playing. I'm glad people who are not familiar with Love Live are enjoying it, since this was primarily created for Love Live fans but meant to be playable by everyone.
"School Idols" don't exist in real life, in the Love Live universe they're essentially students who form self-managed "idol groups" as extracurricular activities.
The Love Live series is Japanese, but this game is originally written in English. Some episodes have been translated into other languages, including Japanese.