Well, to be fair, it was a training sword. At least they apologized.

Holy Necropost, Batman! Anyways, here's the long and short of it:

Tomoko overhears conversation by classmates about recent groping incidents on public transits, and tempts fate by dismissing the possibility of it happening to her.
It happens, but the perpetrator is a kendo stick belonging to a girl from another school. Humiliation ensues.