Update: Turns out Mythbusters has proven you can't explosively depressurize a plane with just a bullet or two. To match that, I've revised the text below.
I just did a little research, and have decided here's how I'd handle things if I were running a game that involved a depressurized airplane. I'll be writing this as system-neutral, so feel free to insert saving throws for endurance rolls, attack penalties for fatigue, etc.
Warning: Such a scenario has a pretty good chance of resulting in a TPK.
Somehow, the plane experiences explosive decompression. Maybe there was a bomb on the plane. Maybe Desmond forgot to push the button. Maybe aliens came to probe you. Maybe the plane was hit by rocket or really large calibre machine gun. Maybe a Megashark jumped out of the water and bit open the fuselage. Whatever the cause, explosive decompression is happening, taking a half second or less to rip the plane wide open.
First, I'd make everyone roll an endurance test. If someone knew the decompression was likely (ie: they saw the alien attack vessel) and indicated they were exhaling, I'd let them make some sort of reaction roll. Success on it would indicate they emptied their lungs before the cabin decompressed, and were safe from the first couple of endurance rolls. Explosive Decompression of the plane takes less than half a second, so it's not something you can do in response to noticing the hole. Anyone who failed the endurance test suffers fatigue penalties for the rest of the scene, and a small amount of actual damage. If they really blow the roll, they pass out.
Then, and only then, do I mention that the fuselage just ripped open like a 500-pound bomb went off, and NPC extras are hurtling out to the dooms. Anyone who specified (before the disaster) that they were wearing their seatbelt is safe. Everyone else must make a dexterity check, probably modified by what's in their hands, whether they were standing or sitting, etc. Failing that check means you're off your feet, and clinging for your life - and must spend your next action regaining your footing. Botching that roll means you're in a freefall. Time to make a new character.
Oxygen masks drop from the overhead bins. There's no longer any force emptying the cabin, but there is a hell of a wind near the hole that might still take Dex checks if your close to it. We're gonna assume a competent pilot who starts taking the plane down gently - if the pilot's dead or an idiot, the TPK is all but guaranteed.
If for any reason people want to start taking competitive actions, we'll roll for initiative. If it's just PCs, we'll probably let everyone take actions, one at a time, roughly simultaneously.
The correct action on your first turn is to secure your oxygen mask. Anyone who does something else (including regaining stable footing from having been nearly sucked out of the plane) must make another endurance test to resist further fatigue penalties. Of course, once you've put on a mask, you're pretty much anchored to a location and can't dodge very well, so let's hope there's no fight going on.
Of course, that's not much fun, so let's assume there's at least a scramble to see who gets the parachutes. If the game doesn't already have rules for pushing others around on the battle grid, I'd state that anyone who's not in a mask can forego damage on an attack to throw someone 5 feet closer to the hole. If they're already within 5 feet, they're now gone. Suffice it to say, you don't want to botch that attack roll.
Two full rounds after the depressurizing event (sooner if the system uses really long rounds), a thick fog forms inside the plane. Like the contrail following a jet - except inside the plane. Visibility is shot, and attack rolls are penalized until the plane drops altitude and speed.
Should make for an interesting fight scene. Like I said, the potential for Total Party Kill is pretty high. With that in mind, it might be a good idea to craft the scene leading up to the struggle in such a way that the best solution is to avoid blowing a hole in the plane in the first place... but what are you gonna do, negociate with a giant shark that can jump 20,000 feet into the air? I didn't think so.