Mayuri is WITHIN the school building?!

Mayuri is WITHIN the school building and just talked to Suou through the wall! What the...between all the possibilities this wasn't even something I was considering possible. The voice was muffled by the wall so it could've technically been someone impersonating her, but Suou recognized her so I'm going to trust the gut instinct of Suou here.
Mayuri seemingly wanted Suou to forget her and move on with her life, but things aren't going down that easily now that Suou that her beloved is within reach. The questions I have just exploded though now.
The school clearly said that she left the school, but that was a lie in which even the Dalia participated into. What else are they lying about? Because clearly things are a lot more troubling now. That explains why Yuzuriha was so reluctant to talk about this topic to Suou, she probably knew something and was instructed to keep silent...but by who? And for what reason?