Not just upper class. My neighbor's granddaughter is learning Mandarin, been studying it for a few years. They're middle class white folks, neighbor's daughter and family live in Arizona. I thought Spanish would make more sense in Arizona, but they think Mandarin Chinese is a better bet. My wife is Chinese-American, fluent with Chinese, but our granddaughter is learning Japanese (mostly due to her also learning Karate). Good to know other languages.
A lot of upper class parents want to help their children be prepared for the world of the future. Why would they not want their children to learn the language of the most populous country in the world?
Studying ANY language is extremely beneficial for the brains development. I would suggest that a few semesters of Latin as more beneficial for English speakers than learning Chinese. The rationale for this is that Latin helps you understand English better, and this is the biggie, although more people speak Chinese than any other language, a WHOLE lot more people speak English as a second language. So maybe reading, writing and speaking English well is more beneficial for a better life than speaking rudimentary Chinese.