SOLVED. YA Horror novel, group in tunnels, gets lost, tunnels possibly under their school. Spoiler ahead. [s]

[Natalie]

All I can remember is there is a group of teens/kids in some tunnels (or something like tunnels - maybe under their school?) and they get lost. I first read it in 2005/2004.

SPOILER there is a red cloud or mist that picks up and kills one member of the group. There might be other dangers too, but I can't remember.

[Kym]

Trapped by R.L. Stine?

[Natalie]

YES! Thank you!