The episode starts off nicely, with Starlight and "The Side Four" so to speak, being rewarded with medals of valor. However, Discord, the sneaky little $@#%?!, makes Twilight feel insecure about her teachings to Starlight Shimmer, driving Twilight to the point of asking for advice from Princess Celestia. Usually Celestia is the best possible pony to have in a time like that, but she (accidentally on purpose) makes Twilight think that she needs to graduate Starlight and send her off, just as Celestia had with Twilight. Twilight ends up announcing that Starlight had graduated, but they both agree that they're not ready to separate yet. Amid good puns, emotional teaching, and an overreacting purple alicorn, the episode is an important installment and a great way to kick off the new season of MLP:FiM. 

There are three main points of the episode analysis, which I will break down. 

1. Twilight was already worried about her teachings.

As we see in the beginning, Discord plays some mind games with Twilight and eventually makes her insecure about the roster of friendship lessons she has prepared for Starlight. As much as we would love to blame Discord for her (supposedly unfounded) fears, the real reason is buried under there: Herself. Twilight Sparkle is too strong of a pony to be shaken by Discord’s petty tricks. In her heart, she believes that her lessons will only be good material for another few moons, and then Starlight will be ready for more. 

2. Celestia is trying to teach a major lesson here, people!

This very well could also be one of Princess Celestia’s cryptic lessons. For instance, she emphasizes how it is Twilight’s decision whether to send Starlight away. That in itself is evidence somewhat that Twilight is being taught a valuable lesson about teaching! As per frickin usual, Celestia is doing it in what's practically code. Celestia is trying to make Twilight a better teacher so that she can make better choices about Starlight Shimmer. 

3. Starlight is also worried. 

This should be rather obvious as a bright unicorn such as Starlight Glimmer would be expected to be thinking ahead of even her teacher. Starlight knows that little friendship lessons can only be actually useful for another maybe year, at a stretch. She also knows that Twilight was sent away, and she doesn't want that just yet. She is hoping instead that Twilight had prepared more advanced lessons that would further educate her in the subject of “The Magic of Friendship” and help her learn more life skills.