I was wondering the same thing. When Harry told Bellatrix that Voldemort's father was a Muggle, she was rather distressed and screamed that Harry was lying. You have to wonder who among his followers really knew/knows the truth.

We don't even know what he told his classmates - almost certainly that his parents were dead, but we don't know that he told them he lived in a Muggle orphanage. I can't see him sharing personal information regardless. And really - he was left in the orphanage by his pureblood witch mother (though he thought it was his father who had been the powerful wizard at the time), so growing up in such a place would not necessarily mean he hadn't been a pureblood and taken in by ignorant muggles when his parents died. The headmaster and professors obviously knew, but...