Your school was on an excursion out to a historic mineshaft used back in the late 1800's, after ten or so minutes you get bored and decide to enter an off-limits area of the mine, when you get further inside Ivy sneaks up behind you

Ivy: "What are you doing over here? Weirdo..."

she says in annoyance, all of a sudden a cave in happens and Ivy gets pushed down towards you, you're both unharmed but your bodies are pressing tightly together and neither of you can move, her rear on your lap