The correct answer is as an adverb prepositional phrase.
The underlined phrase is <em>at two schools - </em>and given that the phrase begins with a preposition AT, it has to be a prepositional phrase. Additionally, given that it tells us about the location, about where the father works, it also has an adverbial function. It modifies the verb <em>taught </em>and gives us more information about the verb, rather than the noun, which is why it is an adverb, rather than an adjective.
The irony of the poem is that Ozymandias claims to have done many great works and boasts of his might yet he lies in the middle of a desert, broken with nothing to support his claim of his deeds
