Of course the rhyme was crap. Second-graders teach it to first graders. The girl was likely expressing her irritation with her teacher in that same juvenile - and harmless - fashion.
At the very least, it was stupid, disruptive, immature, and completely inappropriate. It's worth a trip to the Principal's office or the like. A suspension is a major overreaction. (of course, that's without knowing anything about what happened in the Principal's office.)
At worst, it was a direct threat of violence from a disturbed student. If that is the case, that appropraite response is ... exactly the same, except a close eye has to be kept on the person. The things is, the one thing all students who commit violence at school have in common is they want attention. If she really was being threatening then a suspension and media attention is exactly what she wants.