Two co-workers can make the same salary and spend it in different ways. I think if the salary is, say, $80k-$150k most would consider them both middle class. One can live in a tiny, fully paid for house and save, and the other can spend every dime as soon as it is earn and in fact look more ‘middle class’ to the outside world with cars and a bigger house.
I don’t think it has anything to do with ability to pay for college when the time comes.