This page of the guide to Grand Theft Auto V has a walkthrough for side mission / activity - Flight School. Our GTA 5 guide explains how to unlock the flight school and how to check your piloting skills.
Flight school - general information
Way to unlock: You need to complete Main mission 30 (Scouting the Port) and choose the Offshore attack towards its end.
Playable characters: Michael
Additional information: This one is not a fully fledged mission, but rather an additional activity. If you, at the moment of completing the main mission connected with the examination of the freighter, Michael's flying skills were at an insufficient level, during the conversation with Trevor he will learn that he needs to work some more on it. Trevor will suggest visiting the flight school.
The Flight School is located in the place shown in the screenshot, i.e. in the Southern part of Los Santos, right next to the airport. Go to the flight school and start completing the exercises. Note - you can find a detailed description of the Flight School in the GTA V - maps and collectibles guide.
Increasing the flying skill
Keep completing an exercise after another, best until the moment when Michael's pilotage skills are developed to the maximum level, i.e. to 100 points. After you complete the Minisub and Cargobob missions, an option to start a new heist should become available.