As the former Bearers of Harmony and future Council of Friendship prepare to take the reins of the nation, something has to give. In this case, "something" is their positions at the School of Friendship.
This leaves said school's new headmare in a bit of a predicament. After all, the school now has two headponies, one counselor, and zero teachers.
At least qualifications were never a big concern.
Rated Teen for more risqué innuendo than the show could get away with.