Was it ever made explicit what happened to Muggle-borns during Voldemort's brief moment of control over the Ministry of Magic? I had a quick look over the chapters where the Trio break in to the Ministry, and while someone gets threatened with a stay in Azkaban and someone else is told they'll get Kissed if they resist questioning, there's nothing much I could find about what exactly happens. There's a few students that we're told went into hiding - what about the ones that got caught?
Would it be possible for a Muggle-born student, with no real connection to the wizarding world when they're outside of it, to show up to Platform 9 3/4 or even Hogwarts itself, just to be quietly led away and taken off to wherever the Muggle-borns were kept once they were caught? Would it be further believable, then, assuming Muggle-borns were sent to Azkaban and that there's some kind of similar avoidance of putting underage people in for-reals jail in wizard culture, that the young witches and wizards Harry sees writing out the anti-Muggle propaganda pamplets for Umbridge might have been Muggle-born students put to work doing something menial and demoralising?