We don't know
We know that Crouch Jr. did indeed use this strategy, by his own admission:
“Who put your name in the Goblet of Fire, under the name of a
different school? I did."
—Harry Potter and the Goblet of Fire
We can guess from his previous quote that he used a Confundus Charm to make the Goblet believe that four schools competed.
We know that Dumbledore must know, though:
“Anybody wishing to submit themselves as champion must write their
name and school clearly upon a slip of parchment and drop it into the
goblet,” said Dumbledore.
—Harry Potter and the Goblet of Fire
Thus the name of the school must be written on the parchment.
Automatically, it seemed, Dumbledore reached out a long hand and
seized the parchment. He held it out and stared at the name written
upon it. There was a long pause, during which Dumbledore stared at the
slip in his hands, and everyone in the room stared at Dumbledore. And
then Dumbledore cleared his throat and read out —
“Harry Potter.”
—Harry Potter and the Goblet of Fire
But to my knowledge, nowhere in canon is it mentioned what that school was. It could have been any real school, or perhaps even a fake one.
rts'; ) DROP TABLE Students;
"... credit: XKCD