Same thing I would say about a boy who drinks alchohol in a non-mitzvah setting. (Mitzva setting being a Shabbos kiddush, a vort, ect)
It speaks volumes about the person, and would be (for me at least) a serious bellwether.
To be fair, the poor kids might just not be bright enough to know what they just did was wrong, so I would reveal the reason you are dropping the date (if you choose to do so).
It might be a wake-up call for the offender.