The adverb phrase that describes "will see" is "after school". This is an adverb phrase that describes the time: when does the action of the verb occur.
An adverb phrase is any phrase that acts as an adverb - that is, it describes or modifies a verb (most often), another adverb, or an adjective.
In this case, we can easily see that this is an adverb phrase, by replacing it with a common adverb, for example - later.
