Lemon Zest is pretty lucky for a pony who just got fried to death. She miraculously comes back as a ghost and stumbles on a haunted school where she can learn the basics of the afterlife. The fee of tuition? Recruiting more ghosts to school.
Now that the link between the human world and pony world is constantly open through Twilight's magic, students (whether by accident or on purpose) will wander to the other side.
In an alternate timeline, somepony else got to the Mirror Portal before Sunset and stole both it and Celestia away. Acting Princess Cadance is willing to give Sunset a second chance, and hopes to have Sunset's help with a matter of national security.
Lemon Zest and Sour Sweet decide to open up a lemonade stand one day. As their first day in business drags on, they tackle some pretty senseless customers.