According to the official definition, upper class means being in the top 10% of earners. And for a couple without kids that’s somewhere about 6k, and a teacher makes easily 3k.
That’s the thing is, the average employee is pretty poor, so even such moderate incomes are well within the highest percentiles.
You simply have a different understanding of upper class, that is in my opinion too narrow, since this means a finance bro making half a million a year is not upper class.