My daughter did this years ago. I think they purchased the names from NJHS and the TIPS(Duke Talent Program) - all of the kids in her group were members of one or both.
It is very expensive, and the chaperones vary in their capabilities - some just seemed to look at it as a free trip to Europe. One of them ended up with a severe ankle sprain while on a glacier because it was unsafe (stepped off the marked trail to take photos). It could have resulted in a more severe injury to a student.
Her home stay family in Germany more or less used the two of them as a babysitters, which I guess is pretty close to being a typical experience for the oldest daughter in a family with young children.
The family did give them one evening off, but just dropped them off at an amusement park and told them which bus to take to get home - they were between 8th & 9th grade.
One of the boys had to be removed from one home because he wasn't getting enough to eat. Many of the kids were not mature enough for this, according to my daughter.
Was it worthwhile? I think any travel experience is valuable. Would I do it again? No. For the same amount of money I could have gone with her, but I still had a preschooler at home.
I would ask some teachers, etc if they could recommend another travel agency that specializes in student tours. My son took one to Europe in 10th grade (10 years later) and it cost less. I saw many of them (student tours) while traveling in Greece & Russia.