Pete muttered a few curses in a few languages, as creative as he could get with them, then sighed and squatted, tilting his head at the rune.
"Look," he said, jabbing his wand sharply in the rune's direction. "The long and short of it is that we simply do not have time for this right now. So." He muttered a few spells at William's scrap of pocket, doubling and then doubling again the fabric, and then changing the structure of it until it was thick, woolen, impermeable fabric. Another irritably muttered spell had the fabric wrapping the rune up until it was a wad of gray fabric, harmless to the touch.
"We can deal with this later," Pete said firmly, picking up the whole wad and jogging over the to the narrow window against the far wall of the bathroom. He hopped up, got a hip on the window and groped around outside until he could super-sticky the wad of fabric to the outside wall of the castle. It should be safe from prying young hands out there. Even prying young Ravenclaw hands.
He cast a few more spells at it, making it generally invisible to anyone but himself or William, making sure it would not be damaged or let loose by the elements, and then hopped back inside. "Zombies first," he said, striding irritably towards the door. "Zombies first and then we are seriously going to have to figure out who has their balls all in a knot over the school reopening."