Here's why:
* Predicate: The part of a sentence that tells what the subject does or is.
* Complete Predicate: Includes the verb and all the words that modify or complete the verb's meaning.
In this sentence:
* Subject: All visitors to our school
* Verb: must report
* Complements: at the office (tells where they must report)
Therefore, the complete predicate includes the verb "must report" and the phrase "at the office" which completes the meaning of the verb.