The insidious thing, surely, is that getting in is only the beginning of the story - difficult though it is!
I'm in senior management in a field that is very traditionally elitist, and while I'm white my background is very much rural working class.
I "pass" quite well as a member of the educated upper-MC but only in passing. As in: I have the right voice and accent, know the right things. Nobody would ever suspect I hold a 2:1 from a very third tier uni just by seeing me run a meeting. Nor would they suspect me of knowing a lot more about sheep than family silverware.
It also, arguably, "helps" (although in other ways it really doesn't) that I'm a woman in a very male dominated field. A lot of my differences are simply attributed to my gender by my male peers.
Still, though, it's different for us. I've seen a lot of people a little like me get a foot in the door but fail to thrive in this kind of an environment. If I had to pin it down to one thing I'd say it'd have to be that seemingly natural confidence that we lack. It convinces others they should have confidence in you, too, and if you lack it so will everyone else. I genuinely believe that learning to fake this has been what has made it possible for me, personally, to do well in this kind of environment.
Point in case: we recently hired in a new bloke - typical public school type - at a level below mine. He confidently gave his into in our "get to know" meeting and then went on to immediately ask for my support to get him promoted. I was sitting there thinking to myself "mate, it took me three weeks of fretting and half a bottle of wine for courage to utter the sentence "I guess I'd like to be put up for Director" - to my boss, whom I had known for years, and who'd been pushing me to take the plunge forever.
It's insidious because the system is self-preserving in so many ways.