[Benzgirl]

Look very closely at the agency that is placing them with you. When I was in high school (ok, very long ago), American Field Service was the best and Youth For Understanding was next.
There were many organizations that were not very well run and one of my friends took in a student from Finland from one of them. It was a nightmare. The student drank a lot and told her host-mother that she wasn't going home at the end of the year because she was going to marry her host-brother. If I could remember the name of the agency, I would tell you.
Talk to others that have hosted students from the organization in which you are dealing