MetaphorThe classroom was a zoo
The room was noisy, wild and out of control.
When the teacher stepped out, the classroom was a zoo.
Remember: The class doesn't just seem 'like' a zoo; the sentence says it WAS one. That direct 'is/was' makes it a metaphor. Informal.
Short lessonSimiles & metaphorsTwo ways to compare things so a picture forms in the reader's mind.