As I mentioned in another thread, I'd be very wary of a language school in a 3rd/4th tier. I've been to Yuxi, have a friend who is married to a woman from there -- I attended their wedding, which was in Yuxi. I can text some friends in KM and see if anyone has heard of this school.
Yuxi is where the Hongta tobacco company is headquartered, so there's a lot of money there, but the people there have this kind of provincial money, Saloonies will know the type -- fancy cars, gold chains, chain smoking, but very little education, which tends to be a pretty obnoxious kind of wealth.
Did their ad state whether they can get you a proper visa and residence permit or not? That would be my main concern. Yunnan has a lot of fly by night sort of schools that take advantage of the fact that the province is full of backpackers and hire people who are just passing through. With a little known training school in a small town, I'd want to be 100% sure it was all legal. If they even mention coming over on a tourist visa I'd drop them immediately.