Family was probably on the edge of lower-middle and middle-class. First person in the entire extended family to receive any kind of advanced degree. They do place high value on education and helped some while I was in school (plane tickets home, car repairs). I drove the car my grandparents got me as a graduation present, which was pretty reliable so that was a huge help. If I was in a total bind though, I do think family would have pitched in to help out though thankfully it never came to that. I think the biggest help though, was just having been instilled with a relatively low-maintenance lifestyle. Food on the table, safe place to live, a car that works well enough to get me from A to B reasonably reliably, some peace and quiet and I'm good. Lifestyle has inflated a bit now that I'm further along, but try not to lose sight of that.
I was single for most of grad school, though got married near the very end. Wife (at the time) earned 40k which was perfectly livable where we were. Though shall we say....spousal differences in spending habits were an ongoing negotiation early on and I felt a lot more broke as 2 people living on 60k than 1 person living on 20k when we first got married
Still no kids, though we had quite a few folks with kids in my program. I imagine that would change things dramatically - can't fathom the childcare costs given most had spouses with "normal" jobs that were not particularly high-paying.
Curious what your hypothesis was when asking about this. I could see two different pathways. A higher-income background could certainly mean more more confidence in a safety net and potentially family support to help with expenses. However, I could also easily see people used to a more luxurious lifestyle really struggling to dial it back and live on ~20k.