What I could remember is that a school had a history of an entire class going missing. Some kids found a secret entrance in the school (I think it was an elevator that went sideways instead of up and down) that lead to a sealed-off classroom where they found the missing class and teacher.

The entire room plus the students and teacher were gray. The longer the new kids stayed the more they turned gray too. They were going to be forever trapped in that gray room with the missing kids but one girl realized her red lipstick had not turned gray yet and they were able to draw a door to escape.