When Ron meets Harry on the Hogwarts Express in Harry Potter and the Philosopher's Stone, they have this conversation:
“What House are your brothers in?” asked Harry.
“Gryffindor,” said Ron. Gloom seemed to be settling on him again. “Mum and Dad were in it, too. I don’t know what they’ll say if I’m not. I don’t suppose Ravenclaw would be too bad, but imagine if they put me in Slytherin.”
“That’s the House Vol-, I mean, You-Know-Who was in?”
“Yeah,” said Ron. He flopped back into his seat, looking depressed.
- Harry Potter and the Philosopher’s Stone, Chapter 6 (The Journey from Platform Nine and Three-Quarters)
So Ron apparently knows Voldemort was in Slytherin. But at the end of Harry Potter and the Chamber of Secrets, it is made clear that Voldemort's real identity as the brilliant (Slytherin) student Tom Riddle is unknown to most - in Dumbledore's own words:
‘Very few people know that Lord Voldemort was once called Tom Riddle. I taught him myself, fifty years ago, at Hogwarts. He disappeared after leaving the school … travelled far and wide … sank so deeply into the Dark Arts, consorted with the very worst of our kind, underwent so many dangerous, magical transformations, that when he resurfaced as Lord Voldemort, he was barely recognisable. Hardly anyone connected Lord Voldemort with the clever, handsome boy who was once Head Boy here.’
- Harry Potter and the Chamber of Secrets, Chapter 18 (Dobby’s Reward)
So how could Ron (as well as, I guess, most wizards) not know Voldemort's real identity yet know the actual House he was in at Hogwarts?