It really varies. There are quite a few laws regarding as to what is a firearm, replica firearm and airsoft firearm.
There is a useful thread here:
It's long, but it will give the detailed info about airsoft when it comes to legal issues.
Eg. For instance, if one of the kids modifies their shooter to fire at 500+ FPS, he will be in a world of shit if discovered, since by law anything firing at that velocity may be considered a firearm.