I dont know shit about japanese geography but maps show me that yushima is right by kanda shrine and akihabara where μ's happened, so a school near yushima could technically be otonokizaka right?
and then meeting chun sensei happened in april, and polka has this post from april. and if school idols are currently dead in this timeline then that school becoming popular from is even more likely to be otonoki.
so i think polka found out about school idols somehow and how they saved otonoki, then was so excited by it she trespassed onto otonoki's grounds, and met kotori who's now a teacher and told her about the love live tournament

basically, μ's bros, we are fucking back.