My little headcanon: :p
 
Twinkle did not get into Celestia’s School For Gifted Unicorns because she failed the entrance exam. That’s why she ended up as the sarcstic blogging NEET we see in T-Brony’s comics.

From the artist’s tumblr:
 
“Outshined  
I guess the backstory to this pic if it were to have one: Twinkleshine became jealous of Twilight Sparkles’ success. Though she too, was able to make it into Celestia’s school for gifted unicorns, Twinkleshine always felt as if she was living in the shadow of her classmate. Her hard work and achievements always paling in comparison to the gifted unicorn. Many years later, once news had reached Canterlot about the Elements of Harmony and their defeat of Nightmare Moon, Twinkleshine heard that Twilight had become the Element of Magic. She now spends her days locked in deep research at the Canterlot archives in hopes of one day surpassing her rival.”

Twinkleshine is not quite as motivated as Twilight in learning to use magic.