[quote=@ERode]
the commonfolk can totally be outnumbered in institutions that favour the noble or the wealthy.
[/quote]

This, pretty much. I was going to make the whole "debut spar" thing into sort of a showcase for that. Candidates from powerful, established families are welcomed first after a basic exam, and then they serve as opponents from those who don't have any such reputation to vouch for them, giving them a chance to prove themselves... which also gives the nobles some "easy" opponents to show off their own skills by trouncing the incompetent. However, in that sense, the deck is sort of stacked against the commoners, since they actually have to show that they can hold their own in a fight one way or the other in order to get noticed. The nobility just sort of has to show up.

That's still not to say it's untenably hard to get accepted as a commoner, because the knights do generally accept a lot of candidates and then cull the herd as they go -- it's just they have to work a bit harder than most, so there's probably fewer of them as a result.