Starlight Glimmer is a sneaky bitch. She couldnt have taken over that one town without making plans to spread out. She may have some outside connections that we dont know about. Its possible that she could have started by convincing other ponies to work for her. Maybe she has connections with the human world. The principal of this "Crystal High School" could be working for Starlight Glimmer. The uniforms lead me to believe that the school is indirectly under the control of Starlight Glimmer. Now it could just be another one of those schools in which uniforms are required, but this is more fun to think about.
With human Twilight under the control of this school, she could be out to destroy the humane six, whose pony counterparts interfered with Starlight Glimmers plans. Now this would only solve that problem in the human world, but it could possibly be done in the pony world. The portal that we see in the trailer opens up to what looks like the location in Appleblooms dream. But how could a portal open up to a dream? I may be jumping the shark here, but it might have to do with next weeks episode. The synopsis states: "Princess Luna sends the Mane Six to find a magical force that turns dreams into nightmares." Maybe this force could come from Starlight Glimmer? We know that she has dedicated time to learn powerful magic spells, maybe she figured out some spells to get into dreams and to interfere with them. We all thought that voice talking to Applebloom in that previous episode was Starlight Glimmer, but was proven wrong later on. Or were we? Could it be that Starlight Glimmer manipulated Appleblooms dream to the point of taking the form of Princess Luna? Is this why Luna wants to send the mane six to find this magical force?
I'm probably overthinking this, but speculation is fun. Let me know what you think. Perhaps you may notice something that I have missed?