While Cheerilee is off sick and the Mane Six are off on an adventure, Sweetie Belle's two aunts, Strawberry Kiss (Her mother’s sister) and Sweeten Berry (Strawberry's wife) are teaching the class. For their lesson it's on things that are cute and adorable, there's two rules that the class had to follow: All the girls have to wear dress or gown and the boys have to wear suits and always have a teacup in case it's teatime!

Headcanon:

The events of Season 8-9 (With the exception of The Washouts, Sounds of Silence and Best Gift Ever) and Rainbow Roadtrip are not canon.

Instead of being manifestations of the Tree of Harmony, Strawberry Kiss (The pink pony with the yellow dress) and Sweeten Blackberry (The purple pony with the pink dress) are real are also the aunts of Rarity and Sweetie Belle.

After the School of Friendship gets destroyed by a storm and landside, everycreature helped to upgrade the Ponyville Schoolhouse and made it more bigger. With the classroom expended, more bookshelves, a new paint job for the schoolhouse building, more lessons and more games for kids like buckball and all sorts of fun things. Twilight and her friends now share the lessons with Miss. Cheerilee. And sometimes if they’re ill or busy somewhere their parents or the Background Ponies (Like Bon-Bon/Sweetie Drops, Lyra Heartstrings, Octavia Melody, etc) would fill in for them.