Not research, but in Sociology 101 I picked up that the US have a traditionally high upward social mobility and even poor kids can go to college (if only through student credit) and have a good career afterwards. I've heard that recent financial crises, and the bad job market prospects of liberal arts graduates have changed this somewhat.

In Germany, tuition is almost non-existent (usually some fees of maybe 150€/semester, although a few years back there was a phase of actual student fees of - still laughable - 500€/semester), and if your parents don't make much money, or if you have many siblings, you can even get a half-credit/half-scholarship from the government (AFAIK even without interest, called "Bafög"), so especially for poor kids it's as easy as nowhere else in the world to go study. In smaller towns you can also get a student dorm room for under 200€, and overall living (esp. groceries) in Germany is quite cheap.

Yet Germany has a very low number of working-class students, for whatever reasons. Usually I think it's the parents' influence (but don't ask me why).

We also now have student credit now, in case you don't qualify for Bafög money, but unlike consumer credit which has become quite common, student credit doesn't seem to find many friends. People don't want to go borrow 10-30k for a college degree, but they'll happily finance their 25k cars or their 300k+ condos/houses.