With Sora ending up failing the exam, what happens to him? Where does he go? Those are my questions.

Well, I have a feeling that another side game (based on Sora entirely) is gonna come after KH:3D comes out...

Nah, Tetsuya said that, after he's done with Final Fantasy Versus XIII, he's going to be starting on Kingdom Hearts III.
I doubt they'd make another side-story, because that'd slow down KH3.

Yeah I think three side-stories are more than enough, especially when they each take place in a different timeline.

Namura said there'd be four in an interview. We still have yet to see the story of how Mickey got the Kingdom Key D.

Four?

Wouldn't this supposed Birth by Sleep sequel be what he's talking about?

In the Director's Secret Report XIII, a special handbook that came with Final Mix +, he speaks of four untold stories.
"Presently there are 4 main untold stories to consider: "the period of the King's absence", "the period of Riku's absence", "Roxas's time in Organization XIII" and "Xehanort's past". In this case, the story of "the period of the King's absence" is set in the realm of darkness. I am examining a way to tell these 4 stories so I might be able to find a way to tell them soon"
Now Roxas's time in the orgainization and Xehanort's past have been spoken of, and a very brief part of Riku's absence was shown in Re: Coded. However no game thus far takes place in the Realm of Darkness, or has King Mickey as the player character.

If you don't believe me the transcript can be found here.

Did he specifically say that all four of these stories were to be told BEFORE the release of KH3 though? Or just that they had to be told eventually?

He said that all four needed to be known in order for 3 to make sense, but I don't have any evidence showing whether it's before or after.

Well, remember, there was:
-Kingdom Hearts 358/2 Days
-Kingdom Hearts Birth by Sleep
-Kingdom Hearts Re:Coded
-Kingdom Hearts Dream Drop Distance

Those were 4 games, so...
I think the "Period of the Kings Absence" were the events of Kingdom Hearts I. He did go on an unknown adventure during that time didn't he?
Although you don't exactly get to "GO" to the Realm of Darkness, there are several cutscenes taking place in it.

However that period of time is almost completely unknown to us. Unless, of course, you can tell me how he got the Kingdom Key D.

The story people are talking about is during KH1. He's talking about whatever mickey was doing during that time period.

during the secret movie of BBS:FM, they show clips of unshown things, especially the part where it shows Kairi in Twilight Town for some reason, and Riku after Kingdom Hearts. I believe BBS: ~Volume 2~ will be about Aqua's time in the Realm of Darkness, which was also during Mickey's time in the Realm of Darkness (Aqua came out of the RoD after KH2 was over, as she just met Ansem after he lost his memories).

So BBS: ~Volume 2~ be the last of the sidestories.

Aqua never came out of the Realm of Darkness. She is still there to this day.

And to whoever said 3D counts a side story, I can see why you say that, but it has nothing to do with the 4 said stories. If we get another "side-game" (let's hope we don't) I could either see BBS v.2 with Aqua in the Realm of Darkness and Mickey's story from the end of BBS to the end of KH1. If not, I see a Riku and Mickey game, where it will explore there whereabouts in Kingdom Hearts 1 and Days, as we already know they were present in Castle Oblivion during Chain of Memories.

She was in the Dark Meridian with Ansem which is on the border of the RoD, so making it out of the core of darkness and closer to the light, that was blatantly implied