This could be because I'm a big fan of the Terra-Riku bond, but has anyone mentioned in threads the idea of Riku gradually taking up a massive role because of his connection to Terra? While it's pretty clear that Sora is going to be the one to ultimately save everyone in the end, I really don't think he has the mindset and the proper tools to go and directly save him from Xehanort. Riku, on the other hand, does. He's got a very clear understanding of light and dark, and has managed to succeed where Terra failed. Something could always happen to make Xehanort take an interest in Riku, re-opening the old wounds in his heart the darkness created and as a result, Riku ends up 'finding' Terra.
He might end up going on his own path to get Terra back that could result in a great deal of confusion for a lot of people, and make him end up ditching the exam.
Another option could be along the lines of what many people seem to be saying: Vanitas makes an appearance that will directly harm Sora.
But in the end, I have doubts that either of them are going to end up giving up on the exam. One of the things that struck me about the BBS characters was that in one way or another, they were all very sheltered compared to the likes of Sora, Riku, and Kairi. If Sora and Riku were given the kind of exam that Eraqus gave Aqua and Terra, they'd be Keyblade masters already.
I do think perhaps, that Sora will struggle a bit more on whatever Yen Sid has in mind than Riku. Riku has done a lot of traveling and has dealt with a lot of heavy issues on his own. We know Mickey helped him, but we don't know how much. Sora, at one point or another, has always and constantly relied on friends. Yes, his friends are his power, but they're always quite literally there. I can't remember Sora having to deal with something and relying on his friends, but without them actually being there, in the same way we've seen Aqua do it during the secret episodes while she was in the RoD.
I hope this all makes sense.