In Harry Potter and the Order of the Phoenix
When Hagrid introduces the Gryffindors and Slytherins to thestrals only 3 students are able to see them, as people can't see them unless they have witnessed death. Harry had witnessed Cedric's death at the hands of Voldemort.
Another student was Neville.
I know Neville's parents weren't killed by the Death Eaters, but "only" driven insane. I can't remember Neville seeing anyone else die. So, who did he see die (or was it never revealed)?
It seems safe to assume the third student was just someone JKR threw in.
That's because it would be unlikely to have that many students who hadn't seen death by that time in life.