Alright, friend of mine found this bug with the flight school.
He's playing a PS3 version
Here's the deal - Possible spoilers ahead
At some point of the history, Trevor calls Michael telling him to improve their flight skills at the flight school, because later they will need Michael to do some air assistance on a heist.
The flight school are open since the beginning of the game. Thing is, if you complete the flight school BEFORE Trevor tells you to do it, the later call from Trevor WON'T HAPPEN, and the heist preparation couldn't be started at all.
Just a heads up, do not go for the pilot license too early people...