So, we know that Myrtle was killed by Salazar's Basilisk...
“How?” said Harry.
“No idea,” said Myrtle in hushed tones. “I just remember seeing a pair of great, big, yellow eyes. My whole body sort of seized up, and then I was floating away. …”
...but wouldn't it have been extremely odd to explain to her parents that she was killed by a monster whilst in the loo? He does say this to Riddle:
“Precisely,” said the headmaster. “My dear boy, you must see how foolish it would be of me to allow you to remain at the castle when term ends. Particularly in light of the recent tragedy . . . the death of that poor little girl. . . . You will be safer by far at your orphanage. As a matter of fact, the Ministry of Magic is even now talking about closing the school. We are no nearer locating the — er — source of all this unpleasantness. . . .”
He simply called it a "tragedy", but I'd expect any parent to not be satisfied with such an answer.