"HOW MANY times do I have to tell you, Orion, that I am not interested?" Sienna fumed, currently being dragged around the halls by Orion. "Why am I being dragged around when, clearly, I have better things to do?"

He paled. "Wait and see, Malfoy."

Sienna rolled her eyes at him. Insufferable, over-enthusiastic, Nargle-obsessed freak. 

She gave up resisting, and jogged alongside him. He rolled his eyes. "Pick up your pace."

They climbed down a few flights of stairs, ran into people, Orion motioning to many students to follow them.

The crowd jogged with them towards the corridor near the castle grounds, and Orion shakily pointed to the wall.

Blood.

Sienna rubbed her eyes. The crowd behind them shifted uneasily, muttered and whispered to each other.

"Blood," she breathed. "It's been years since..."

"It's been thirty years, precisely," a fifth year Hufflepuff supplied.

Orion shivered. "What?"

(Now here, I'm going to give y'all an explanation on how it's thirty years. 

The epilogue at the end of The Deathly Hallows starts with nineteen years later. Albus Severus was about to start his first year. 

The last time blood appeared on the walls was when Pottah and his friends were in their second year. So, if we're considering this if they're in their seventh year, it would be five years {for the idiots: 7 - 2 is 5}. Add that to nineteen. We'd get twenty four years. 

And now, Sienna and Orion are in their sixth year. So, we're only considering their ages. Add six to twenty four.

Bam. Thirty years since bloody writing appeared on the walls of Hogwarts. You're welcome.

If anyone has any theories, I'd really appreciate debates in the comments!)

Sienna shook her head. "It's true."

"What does it say?" A first year Gryffindor squeaked in a shaky voice.

Orion swallowed. "No..." He breathed.

Sienna shakily read out the message. 

"Your efforts are in vain. We will return."