What adverb phrase describes will see in the following sentence?

I will see you after school.

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.