There are some things that are appropriate for school settings and some that are not. Where in the world is this school???!!!! If the lyrics truly are not school appropriate that would not be acceptable in any school in which I've worked. Just as there are rules for appropriate dress and behavior at school. Our kids are not to display certain behavior--no PDA, no profanity, keeping certain body parts covered in their uniforms, etc.
As far as saying, as some parents have here, that "kids are going to be exposed to it anyway." Well it shouldn't be at a school sponsored function. Kids also need to be taught where things are and are not appropriate.
You've been to the principal so now it's time to take the lyrics over his/her head and talk to the super of secondary principals about your concerns. Parents and schools are supposed to work together on education and matters of decency and propriety concerning the students. Sounds like the principal is dropping the ball on this one.
As a parent, I did not allow rap in my home except for Will Smith--which shows you how old my daughter and I are. I exposed my daughter to the variety of music I was brought up with from classical, jazz, rock, gospel, oldies, etc. She chose her own favorite style. No matter how the other children were into rap, she was not into vulgar language as we did not use it in our house. So syaing they will be exposed to it anyway is copping out to one's parental involvement and placing "the world" in charge of your child's upbringing. I was not having it. We were aware of what was out there but it was OUR choice as to whether it was to be apart of our household. Just like what is on TV. I'd discuss these thing with my child and what was going on and why certain things were not acceptable.
As a parent I have control over the TV controls.
There are "good" rappers out there so let's not put all rap into one basket. I'm not into rap but I find these decent ones to make my point with my students and they love them!