Well, what I remember is that the show was pretty similar to Skyland, because it happened in the future and characters lived on airships or floating islands, but it was a movie, not a series. The animation was also quite similar, but I'm pretty sure i saw this years before Skyland came out.
When it comes to the plot, I can recall that it centered on teenagers (around 14 years old) that were aboard giant airship (maybe a zeppelin)that acted as some sort of military school, and, a scene that I remember pretty well consisted on the main character and his best friend being ordered to control some turrets and defend their base. The protagonist is really nervous but ends up doing perfect and even taking down a few enemy ships while his friend is killed in the attack. And, later on on the movie, the protagonit falls in love with a girl and everything goes well between them. He ends up turning into a pilot (to become an airfighter pilot was really important to those kids).
The rest I can't remember. I believe it had the word "Sky" on the title, but I'm not really sure. Any help would be pretty much appreciated