This is most evidently a metaphor. The thing to keep in mind when analyzing literature/poetry is that if you see figurative language making comparisons/drawing parallels between two differing things--instances where the meaning is not literal such as how injustice is not really a fire and has no flames--it's a safe bet you'll have on your hands a metaphor.
It was considered protected speech because wearing symbolical armbands to show your opinion is a part of the first amendment rights of speech. The school was reprimanded for censoring and forbidding people from expressing themselves.
