The correct answer is as an adverb prepositional phrase.
The underlined phrase is at two schools - 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 taught and gives us more information about the verb, rather than the noun, which is why it is an adverb, rather than an adjective.
