Would you be just as upset if some other boy were singing about shooting your friends, the school did nothing to stop it, and then that kid ended up shooting his friends? Laughing and goofing off about shooting someone in no way can be considered that the person isn't serious about wanting to kill someone - look at how many murderers laugh and smile during the descriptions of their crimes at trial.
Teachers don't have a magical power to say "to this kid its just a song, to this kid its real." If they did, most of the school shootings would have been stopped as most of the shooters talked about it for quite a while before they did it, and everyone just brushed it off as a kid thing. Even that little first grader who shot and killed a girl a few years ago said he was going to kill her the day before.
I wonder what the lyrics of this song were. Its not one I'm familiar with, but from the two lyrics you gave, there is no way i wouldn't report it, if I heard it.
I'd way till you are calm, and then call the school, explain your view on your son not understanding this, and ask to have the school psychologist discuss why its inappropriate to use such words in any context to him.
*I can point you out schools where a child of his age would have been expelled and/or have to attend special counseling aside of school for such behavior. You should really be consideing yourself lucky that he was laughing about killing people and all that happened was a warning. Or maybe be considering yourself unlucky, because it could have just as easily been some kid who was seriously considering killing others, and enjoying the song because of that, and all the school did was give out a warning. While rare, school shootings do happen, and that other little boy may have found the song fun because he found the idea of killing someone fun.