New spells are fun, and Twilight loves practicing them. An ill-timed sneeze has made her latest spell go a bit out of control, though, and now she's stuck as a tiny kitten, trying to deal with a pony-sized world.
Harry Potter, after a beating by Dudley and friends — with the help of a real gang member — wishes he had somewhere safe to go, and starts crawling home. He ends up in Equestria. The CMC find him. A year later, an owl brings his Hogwarts’ letter!
When the CMC asked Discord to help them attend magic school, he pulled an owl out of his hat. Only he didn't exactly have a hat. Which was okay, since their new school had a singing one laying around. Where the hay was Hogwarts anyway?