If you're in the capital class, you're getting your income from the assets you own. If you're in the working class, you're getting your income from working.
I've heard multiple definitions for a middle class, eiher one that owns some capital in the form of rental apartments or stocks, or that the middle class has a decenr amount of discretionary income.
Personally I don't think the middle class is that useful of a term to make sense of the economy. I also have a feeling that people like the term middle class because it muddies the waters when it comes to understanding the relationship between capital and labor.
Middle class is a state of mind
Having a college education could totally be an indicator for middle class even if most people didn’t have one.