When Diana transitioned from elementary school to middle school, she moved from being one of the oldest, biggest, and most powerful students in the elementary school to being one of the youngest, smallest, and least powerful students in the middle school. This phenomenon is known as the?

Top dog / bottom dog effect.

Explanation:

When students move from the very favorable, safe, and familiar environment of primary school to secondary school, they move from being the top dogs to the bottom dogs in the new social hierarchy. This kind of change is known to cause high school social and academic problems. This phenomenon is known as Top dog / bottom dog effect.

This is known as the Top-dog phenomenon.