At parents of future exchange students you are at the mercy of the agencies entrusting what you have dearest to strangers.
How can you prevent such things from occuring?
It is almost impossible!
However you may demand that you can skype with your child at least 3-4 times per week, so your child can reflect all what he or she may meet of strange behaviors or customs abroad.
Only by personal intervention can you keep your child safe. Do not forget that the agencies are businesses - many of them opening under new names whenever the old name become a burden for them. Other manage to fight the families of the exchange students off by hiring expensive lawyers for the very money they receive to keep the exchange students safe.
Demand online communication with your child in order to keep your child safe!
Exchange students abused in the U.S. (CBS News)