The mirrors shattered - A. J. Hartley

The third book in a gripping fantasy adventure series from bestselling & Carnegie nominated author, A. J. Hartley.The problem with doors is that they open both ways. There are monsters
inside, and some of them are trying to get out
...Mr. Peregrine has been kidnapped! Now it's up to Darwen, Rik, and
Alexandra to rescue him. They'll embark upon a journey that will take them deeper into Silbrica than they've ever gone before, and there, on the other side of the mirror, they'll find the strangest sight of all: an
exact replica of Hillside Academy. This is only the first of many
connections that Darwen and his friends are about to discover between the human world and the world of Silbrica. And as the connections begin piling up, they'll be forced to confront the most horrifying
realisation of all: they've been lied to this entire time.