The teachers would speak mostly in Mandarin. My son came out of Spanish immersion preschool speaking pretty close to native level Spanish (despite neither my wife nor I speaking it).
Congrats! I tried to teach my kids Russian (my wife doesn't speak the language). My big daughter used to understand Russian, but didn't speak it; Later, our second daughter did have some slight delay with language acquisition, so that I stopped trying.
I am not sure that your son will keep the language, once he is out of the preschool. A second language is a very fragile posession, as far as i know...
Keeping a second language is certainly hard; I lost my parents' native language when I was a kid and still regret it.
Since my son left the preschool, he's been doing online virtual lessons with a lovely woman we found in Guatemala to keep it up, and fortunately his new school has a heavy emphasis on foreign languages. They have different Spanish classes depending on the child's ability, and they put my my son in with the kids who are native speakers at home. Our only problem is that the teacher sends communications to (presumed Spanish speaking) parents in Spanish, which my wife and I have to Google Translate!
