Poltergeists and Featherlights
When Myrtle was younger, back before her death, while she was still a nervous Ravenclaw student, she had another problem besides her bad grades, less then ideal looks, and hopeless crush on the Slytherin named Tom Riddle.
That problem went by the name of Peeves.
Yes, Peeves the poltergeist. He was, technically, a problem to everyone. He pulled girls hair, stuck gum in key holes, and if parchment was left unguarded anywhere for more then a few second, he would be there immediately, scattering the pages to the wind.
Myrtle had it a little worse, though. You see, Peeves had taken an especial shine to Myrtle at some point during her miserable school career, and she didn't much appreciate it, because the only form of attention Peeves gave was bad attention.
He did more then pull her hair, he once ripped a chunk of it out.
He put gum between the pages of her textbooks and stuck to the back of her jacket.
Sometimes her notes weren't just scattered, they were obliterated completely.
But it was the day he shredded her homework the morning it was due that was the last straw. They were in an empty corridor; no one wanted to walk to class with silly Moaning Myrtle, as Peeves had often pointed out to her in his teasing. Normally this made her terribly sad and weepy, but today she decided to look on the bright side: this meant that there was no one to hear her snap and start screaming.
And Peeves just stayed there through her vent, a stupid grin on his stupid face, making Myrtle grow steadily angrier and angrier until she gave one final screech of frustration and stormed past, still fuming.
"You look really pretty when you're angry!"
Myrtle gasped, and spun around, but Peeves was already gone.
After being late for class and not turning in her homework, Myrtle got scolded by the teacher, and assigned detention after dinner. She never made it to detention though, because that evening during dinner, Olive Hornby makes fun of Myrtle's glasses, and she's already had such a terrible day that it all built up and she ran away in tears.
In a bathroom she finds solace, and is sobbing and stiffling to her hearts content when the door opens. She hears whispering, or is that hissing? She calls hello in a shaky voice and reaches for the door handle. If Olive is here to make fun of her again, she might as well face it now, she thinks as the door swings open and bangs against the next stall over, instead of later when her face is finally cleared and there are people around.
It's not Olive, she realizes when she sees yellow.
And then she dies.
Only she doesn't realize that at first. When she comes to a few minutes later, she doesn't realize she's a ghost, not until she sees her still body laying stone cold on the floor and in her shock at the sight, she falls straight back through the stall door, where she stays crying until the door opens again.
Olive is the first to find her body, and Myrtle doesn't stick around for long after that.
Then Peeves is the first to find the rest of her.