This is a wonderful little animation you've got here! Everything moves quite fluidly and vivdly, and the designs and rigs look almost exactly like the ones used in the official show. It really does help a lot in making the characters come to life, and helps this excel in the animation department more than a good chunk of the animations on NG.
The main thing I don't like here is the story: not much seems to happen here, in my opinion. Despite this being an animation assignment like you said, perhaps next time you could consider making something a little more "meaty" story-wise, if you catch my drift. It would help a lot with helping this animation stand on something else besides the animation.
Other than that, I've got no complaints. Can't wait to see what else you cook up, MLP or non-MLP!