Final answer:
The complete verb phrase in the given sentence is "is harder."
Step-by-step explanation:
A verb phrase is a group of words that includes a main verb and its related elements, such as auxiliary (helping) verbs, objects, adverbs, and complements. The verb phrase functions as the predicate of a sentence and provides information about the action or state of being.
In the sentence "School is harder in tenth grade, but I still should be able to get high grades," the complete verb phrase is "is harder."