What is while in the sentence The class became hilarious teacher was out?

The word "while" is missing from the sentence you provided.

It seems like you're trying to express something like:

* "The class became hilarious *while* the teacher was out."

In this case, "while" acts as a conjunction that shows a contrast or simultaneous occurrence. It connects the two clauses:

* "The class became hilarious" and

* "the teacher was out".

The sentence means that the class was funny because the teacher was absent.