Well, the kid may or may not be lying. If the kid is telling the truth, the suspension is unwarranted and ridiculous. If the kid is lying, I still think a suspension is a bit extreme. Maybe detention, or requiring her to write an essay on why it's wrong to swear, or something along those lines would be more appropriate.
I went to Catholic school, and in religion class we sometimes discussed hell. Can you imagine suspending the entire fourth grade because we were discussing hell in class?

If the girl is telling the truth, it's the same sort of thing - she was admonishing the boy for swearing to God, and should be praised, not punished.