“We will depart as a class, but when we arrive we will split up into small groups.”
In this sentence, which part(s) is/are (a) prepositional phrase(s)?

Prepositional phrase “as a class” modifies verb “depart,” with “as” meaning in the role of (a class). Prepositional phrase “into small groups” modifies verb “split,” with “into” meaning becoming or producing (small groups). (Note: Preposition “up” also modifies verb “split,” creating the verb phrase “split up.”) However, “when we arrive” is not a prepositional phrase but an adverbial dependent clause, modifying the predicate “we will split” by specifying when, in the independent clause “we will split up into small groups.”