Dread Wood : Book 1 by Jennifer Killick

It's basically the worst school detention ever. When classmates (but not mate-mates) Hallie, Angelo, Gustav and Naira are forced to come to school on a Saturday, they think things can’t get much worse.

But they’re wrong. Things are about to get seriously scary. What has dragged their teacher underground? Why do the creepy caretakers keeping humming the tune to Itsy Bitsy Spider? And what horrors lurk in the shadows, getting stronger and meaner every minute …? Cut off from help and in danger each time they touch the ground, the gang’s only hope is to work together.

But it’s no coincidence that they're all there on detention. Someone has been watching and plotting and is out for revenge …