So, I'm running a game online with some friends from out of town via email.  I started them out on the pre-generated adventure Night Fears, in which the characters are all high school kids trying to spend the night in a haunted house.  By the end of the scenario, they decided they liked their characters enough to keep playing with them, and I've been running solo GM-wise since then.

Since all the characters and events revolve around the high school, we're trying to keep the tone of the game light-hearted and fun, without bringing a lot of the dark, gritty elements into play.  The scenario I'm running, the players have run into a couple of fellow students who are focused practitioners, who have just been running amok with their new-found abilities.  They've managed to glean that one of the teachers at the school is a wizard who is gathering a following among the students and is teaching magic.  They just don't know which teacher it is, and are trying to find out.

In a recent conflict the party managed to capture one of the apprentice wizards, and want to interrogate her to find out who the wizard-teacher is.  While trying to find a means of interrogation that wouldn't fall into that grim, gritty category, the party's resident trickster/comedic relief suggested tickle torture.  Feeling this was appropriate, the other players agreed.

I, for one, am curious to see where this goes, but I'm unsure how to go about running a conflict about tickling someone.  What stress track would be used?  I'm tempted to think physical, but I could also see how mental stress could be a factor.  What skills would be involved?

Anybody have any input or suggestions as to how this scene would play out mechanically?