Answer:
quickly
Step-by-step explanation:
Adverbs usually modify verbs, but they can also modify adjectives and other words. The function of an adverb in the sentence is to elaborate on the extent of an action, how something happened, or where something happened.
In this sentence, the adverb quickly is describing the rate at which the school year passed. Without the adverb, the reader would only know that the school year passed, but nothing about the speed at which it did so.