I'd like your theory to explain the part where Twilight and Spike physically went through the portal. And the part where she even tested it early on by sticking part of her arm in it, seeing a hoof inside the portal.
And to everyone, I'll also add this exact line from human Pinkie Pie:
"Do you have a twin sister who lives in the city, has a pet dog named Spike that looks just like that one?"
She only referenced where human Twilight may be living and her dog. Did not mention school.