And her "solution" was wrong for a different reason than what everyone else seems to be focusing on: If teachers had a problem with "freaking" they should have passed a rule against that. Checking the undergarments of students who may or may not have planned to engage in the activity would have no effect on the students who did plan to.
The teachers who don't want to chaperone any more, did they say anything about undergarments? If not, why was that the target of the response? Did the vice-principal think to simply tell students that anyone seen freaking at the dance would be kicked out?