I went to China with ECVA for a month last summer. I had already been to China twice on high school trips and had taken 6 years of Mandarin classes so I already relatively had a good handle on the language. I still ended up having a lot to learn and managed to improve my language skills in a way I wouldn't have in the classroom.
I lived with a family in Beijing that had a 6 year old boy that was already learning English in school. My lessons with him quickly became very informal (at his parents' request). Most days we would play or go somewhere with his parents and speak as much English as possible.
ECVA also set me up with language classes during the week, which I would suggest. Even though the other students were a lower skill level than me, it was a great way to meet other English speakers. I ended up becoming good friends with my classmates and hanging out with them a lot.