When Harry and the others used the time turner to kill Sirius in their third grade, they were accidentally "encountered" by Draco. Then there was a problem with the time turner, which took Harry and the four of them to a thousand years ago. They were picked up by someone and later became the Big Four.
Harry Potter: Salazar Slytherin (Quatal Serpent)
Draco Malfoy: Godric Gryffindor (The Golden Dragon)
Hermione Granger: Rowena Ravenclaw
Ron Weasley: Helga Hufflepuff (Ron is a boy! He just likes to wear women's clothes! Because it's true that Helga died trying to save him!) (Druid)
It's not a bit like the return of my giants thousands of years ago, but when they come back it's the Triwizard Tournament, and Harry and the others don't change schools.
Concept: Time is the most wonderful magic