I did try and game this. She leaves early, which I wonder now wouldn't be a fairly common thing. Like, here your underaged magic kid just needs a few years of basic training, then you put them in a "real" school. Especially in a mixed magica/muggle family. I have her like 5 years younger... ? I can't find my notes. Let's say that. So he's a seventh year when she's second. That's a gap with no interaction in most schools. Except for the Prefects and star Quidditch players. I sort of expect by sixth year, Snape was trying to lay pretty low to avoid attention if he had fallen in with the wrong crowd both inside and outside of school. 

If the school is really 1000 (which the numbers don't work from the house-year level, but supposedly) then I can see the younger kids not paying much attention to the older kids in that large of a student body. That's my set of assumptions to make this work. Otherwise it doesn't work. So I can totally see this being a point of disbelief. 

I was relying a bit on insular Magical society ignorance in part there. They are living behind the times on worldlyness too I'm thinking here given how few Indians there are at Hogwarts. 

Ah thanks! That line got added in the last edit. I realized that section was such a good chance for Harry to learn things about himself. And to realize how much he'd defined himself based on being in Snape's phpere. Originally it was mostly drama and a measure of how far things had come for Snape.