This story is a sequel to Without Your Cutie Mark
The Great and Powerful Trixie has her eye on a place in Twilight's circle of friends at Celestia's School... and then suddenly discovers it may be her only chance to stay at the School...
For more of this AU, see: Saving Sunset Shimmer (Note: You could also optionally take a detour through its backstory prequels: Sunset -- Part I and Part II.)
And check out the series, beginning with Not Exactly Friends.