Based on High School Musical, and EG, this is Equestria Girls, Freshman, with some brand new songs.
chapter 1: equestria: many years ago
"But Princess Celestia!" protested Sunset. "Where are you taking me?"
"What do you see?"
Sunset sighed. "I see myself as an alicorn, standing beside you, as a wise ruler of equestria."
Princess Celestia steered Sunset Shimmer away from the mirror. She saw something that Sunset did not see.
Sunset Shimmer spent the next months in the restricted section of the library. It was 4 in the morning and she was reading about the portal, when a shadow, Celestia came up to her.
"ARREST HER!" she shouted.
Sunset kicked out her hind legs and wriggled, trying to escape the guards and finally fought free, galloping into the portal room.
Without 2nd thoughts, she jumped straight into the portal.