Indianapolis Metropolitan police officers were at Perry Meridian Middle School in Perry Township Monday investigating the incident, the Indianapolis Star reported.
The suspect allegedly sent a a 12-year-old girl Facebook messages that said she would kill her by "cutting off her head, putting it on the porch and (scattering) her body parts in her yard," a police report of the incident said.
The older girl also allegedly said she knew "99 ways to kill her and never get caught."
The police report said the suspect had been expelled from school because of her "explosive behavior," and is currently being home-schooled.
It was unclear Wednesday if charges would be filed, the newspaper said.