On Graduation Day, there were explosives scattered throughout Sunnydale High School.
Sometime during graduation they detonated...
The second explosion threw the trophy case against the hallway's far wall, shattering the glass and sending the trophies flying. Many were damaged, including one, um--shall we say SPECIAL cheerleading trophy?--which broke in two as it clattered to the floor.
A blue light briefly flared; when it vanished, a slender wild-eyed woman could be seen howling in long-suppressed triumph.
"Yes! FINALLY, I'm out of that damned metallic prison. I--"
Then the third explosion dropped five tons of debris straight downwards.
One week later, they finally dug her body from the rubble. And if anybody recognized it, they were sure not to mention the fact.
Although they did note how her eyes seemed to follow you wherever you went.