It’s not about “we must protect the children!” from any little bad language, it’s about setting an example as an authority figure, about how to behave decently in public. How can the bus driver then punish a student for flipping someone else off, for example?
No, it’s not a horrid offense, but for those who are claiming that it’s no big deal to flip someone off-try giving your boss the ol’ one finger salute when you go back to work on Monday.