Remember what Celestia said about the mirror? It is a different dimension so the main characters in the human dimension aren't the same as it is in Equestria. The school must have been Equestria so they wanted to place all the ponies in the show into the movie. Some of the background characters in the human dimension I have noticed aren't very familiar to the background ponies in Equestria. I did recognize Cloudkicker, Scootaloo, Apple Bloom, Sweetie Belle, Diamond Tiara, Silver Spoon, Princess Luna, Princess Celestia, Carrot Top/Golden Harvest and some more that I can't seem to remember.