In this AU, Celestia and Sunset have swapped roles.  
 
Celestia was a former student of Princess Shimmer that wanted to become a princess solely for the power it would give her. She became so hungry for more magic that Shimmer dropped her as her student, knowing that just a want for power and nothing else is not what a leader should want.  
 
In an act of rebellion, Celestia snuck into the castle and to a forbidden section with a mirror to another world. When she tried to pull energy from it, she was sucked into this other world and posed as a student at Canterlot High as she waited for the mirror to re-open.  
 
While at the school, Celestia became the head mean girl and tyrannical queen in a sense. Everyone feared her until the arrival of a new student, Starlight Glimmer.