Here's why:
* Verb Phrase: It contains the main verb "joined" and the object "our school".
* Predicate: It tells us what the subject of the sentence does.
Example:
* He joined our school.
* "He" is the subject.
* "Joined our school" is the predicate, telling us what "he" did.
To form an adverbial phrase, you need a word or group of words that modifies a verb, adjective, or another adverb. For example:
* He joined our school yesterday. ("Yesterday" is the adverbial phrase modifying the verb "joined")
* He joined our school happily. ("Happily" is the adverbial phrase modifying the verb "joined")
* He joined our school very quickly. ("Very quickly" is the adverbial phrase modifying the verb "joined")