When Twilight's magic spells and experiments fail, there are always negative effects, right?
Of course there are. But this time, it's more serious than ever before. Twilight thought that after some research, she could, with the help of her friends, harness the mysterious and powerful magic of a storm. At first she succeeds, but an unknown factor causes it to blow up in her face. Literally.
The massive blast caused turns them all into humans, the only reminder of their homeland being a few assorted items representing their race, and of course they keep their colored hair. Since they appear to be teenagers, a native human living nearby takes them in, and since her parents work a lot, the ex-ponies must endure high school. Fun! (Sarcasmmmm)
The ex-ponies have to learn about human society and government, and how Earth works. As they learn to love their new temporary home, they find that this "negative effect" might not be so negative after all.