For what it's worth, a guy in high school was nearly kicked out for having a paintball gun in his car. This was probably back in 99, right before the Columbine thing....
Forget even whether it's illegal or not... what will the court of public opinion say? I think having the gun would be illegal whether or not it's a private school... but regardless, if discovered you'd be arrested. You'd then have to pay to post bail, all of your lawyers fees, etc. Even if at some point the school rules in your favor, you've still spent a small fortune on lawyers just to get to that point.
Think of the boys of the Duke Lacross team. There was an accusation, the press and public ran with it, and a year later they finally have it somewhat sorted out. But all of those boys are broke now after all of their lawyer fees.
Bottom line... you're not in a life or death situation, so it sounds like you're risking your life (in the non-literal sense) for this. Just doesn't seem worth it to me.