First and foremost, you'll all be under the age of 18 (or at least if you're in HS I'd hope you are). You'll want to have a hand on adult teacher overseer/counselor/adviser.
You'll want him to have a VERY active role in the organization. That's going to be the hardest to find, because many teachers just want to go home right after school.
AFTER you get a teacher who's fully in it, you'll want to sit and talk to the principal or some other person in a high position. If the first step didn't stop you, this usually does.
Now, the first 2 steps are usually a road block due to the wording and visual aids used.
Use replica instead of weapon or gun. Put a emphasis on safety. Emphasize that airsoft isn't just a bunch of kids/adults running around w/ guns shooting at anything that moves. Explain that it takes discipline, a level of respect, of course honor, and a level head. You also want to classify this is an organization, not team. You can call yourselves a team on the field, but when talking to anyone you should make it known that it is a student organization, not club. If need be I'll elaborate why you're not and shouldn't classify yourself as a school team.
Now the fact that you are all under 18 and want to possess these weapons WILL arise. No clue what to tell you here dude :-/
IF, not when but IF you get to this point, First off props to you dude, you'll need to make up a waiver/promise for all members of your organization to sign. It will basically say members in the organization won't do anything dumb such as bring their weapons to school or represent the school/organization (and sport) in a negative light. From here things are smooth sailing.
Now I will say, I personally think in this state that a high school with an airsoft organization isn't the best idea. Not saying don't peruse it, but I see a Rosie O'Donnell size (stupidly large) chance of it falling through, but if it takes off you'll have tons of help backing you up, I personally will do anything to help you guys out as much as I can. But I personally recommend you just group up with your friend and make yourself into a generic team. Appoint a leader and second in command, have meetings every so often and go from there.
Again, not trying to discourage you, I'm just a really blunt honest and open person who tends to speak his mind.