Sora needs to be taught by wielders whom are officially labeled as "Masters". When speaking of Master, those who come to mind are:
Keyblade Masters:
- Xehanort
- Eraqus
- Yen Sid
- Mickey
- Aqua
Now let's narrow it down to a list of possible candidates for training Sora:
Candidates:
- Yen Sid: he's available anytime
- Mickey: same as Yen Sid
- Aqua: Mickey and Yen Sid know where she is, so, they can rescue her anytime and ask her to teach Sora
Those who can't and why:
- Xehanort: obviously, he's busy plotting on getting Kingdom Hearts
- Eraqus: he's inside Terra's heart
In summation;
The Masters that can train Sora and/or Riku are Mickey, Aqua and Yen Sid.
But, personally;
I think Yen Sid should be Sora's Master, but there's a possibility that Mickey could be, but under Yen Sid's supervision. And Aqua... I think they're gonna wait for either Sora or Riku to be a Master or Masters and let them confront alone the Realm of Darkness, a place where there are Heartless in every sight.
Also~
Before anyone asks why I didn't list Terra in there, it's because I said...
Sora needs to be taught by wielders whom are officially labeled as "Masters".
Terra's practically a Master, but, theoretically speaking? He isn't.