To understand the true nature of Terra Falls Academy, one need to go back to World War Two, or more precisely its tail end, when the U.S.A. bombed Japan. On top of all the known effects they had, the two nuclear bombs also had another one: Killing all mystical beings living in and around the Archipelago.
And that's just the immediate effect! It also poisoned most of the magical population of the world.
In the hope to live long enough to cure themselves and/or have decedents, all mythological beings moved to a pocket dimension. But just moving wouldn't be enough, they also wanted vengeance.
Their retaliation, was to construct two identical schools. One in the ‘real’ world, linked to the second one in their pocket dimension. Any human entering the first school would become a new ‘monster’ (possibly one from one of the currently extinct species) and be teleported to its counterpart in the pocket dimension.
As to how nobody noticed the Earth-side Terra Falls Academy had never been open in the first place, and wasn't truly abandoned? The answer is quite simple: Mind-manipulation magic.
— – - – —
The current head mistress of Terra Falls Academy was in her office, observing the latest human intruder thanks to the CCTV system.
"I wonder what you'll become, young one." She wondered aloud "my only certainty being that you'll definitively turn into a cutie-pie" knowing he wouldn't be male anymore once he had moved into the dormitory.
The head mistress remembered when she had been in Jayden's place. At first she had been scared of what was happening to her, at least until her change had been far enough to affect her mind too. Then she started hating humanity for what had been done to her new kind. She was the first kitsune since the bombs had exterminated them.
Once she finished reminiscing about her past, she switched cameras, searching if others had entered with Jayden, until she noticed his friends outside. The fact they hadn't come inside was a problem, but one easily corrected. Her nine tails undulating in excitement, she used her magic to send 3 text messages, one specially tailored for each of Jayden's so called ‘friends’.
Who know, with enough luck one of them would become a kitsune husband for her.
… Or a kitsune wife. She doesn't care much as long as its someone she can love, and she is loved back.
— – - – —
Adam, George and Roger heard their phones signaling they received a new message. Since it was simultaneous, they all believed it to be from Jayden. They all felt compelled to open their phones and were surprised to see no phone number attached to their messages. Unable to resist opening and reading them, they soon saw those messages were signed "Kono Mitsune, headmistress of Terra Falls Academy".