In the sentence "As soon as they got to school, the students fed the fish in the classroom's aquarium," what is the underlined part called?

The underlined part of the sentence "As soon as they got to school" is correctly identified as an adverbial clause. An adverbial clause provides additional information about when something happens, in this case indicating the timing of the action that follows—feeding the fish. It begins with the subordinating conjunction "as soon as," which signals that it cannot stand alone as a complete thought and is dependent on the main clause for its meaning.

In this context, the adverbial clause modifies the verb in the main clause "the students fed the fish," explaining when the students performed that action. The presence of a subject ("they") and a verb ("got") within the clause confirms its status as a clause rather than a phrase, which adds further support to its identification as an adverbial clause.