i did flight school very early in the game, as F, as soon as it unlocked. at that time i had no flight "skill"
the helicopter runs were the most frustrating parts of the entire game for me.
because i'm like that, i got gold on every flight school mission before moving on to the next, and finished the whole school before doing any other gameplay.
when you have no flight 'skill' the game won't let you keep the heli stable, and twists it around contrary to how you are trying to control it.
that i think was the source of my frustration.. the time limits for the medals are tight on the helicopter trials, so it was really hard for me to get the dam thing under control well enough to pass.
eventually i did tho
IMO i'm not a bad heli pilot at all - once the game quits purposefully screwing with the controls!
on other playthroughs i didn't do flight school at all - but possibly it will be easier for you if you build up your flight skill before you try them.