I am latest in the manga chapter. So I know the history behind Koro-sensei. But I wonder how can he fly? I mean, I know he is super fast. But being fast doesn't mean you can fly. And he don't have any psychic or esper powers to levitate or fly.
He may be fast to run in the air or something like that. But when he is flying he is not upright, he is laying down like super man.
He may also use his tentacles as propulsion, but as seen whenever he is flying it seems his tentacles are just there.
How Koro-sensei can fly has never been mentioned in the manga or the anime, so I can only explain it based on my inferences from the manga.
Koro-sensei has been referred to as an octopus on multiple occasions in the anime and the manga (eg episode 10 at around 10:40 in the anime). So it is fair to assume that Koro-sensei's body behaves like an octopus.
When you read about the locomotive system in octopi and the like, it's a jet propulsion system that they use to move around in water. Koro-sensei should be able to use similar methods in the air. As all his cells are tentacles1 and he can freely control his tentacles2, Koro-sensei should be able to fly with a jet propulsion system like the octopus.
1 Chapter 140, page 14.
2 Chapter 137, page 17.
