Castle was released Monday from the Tri-County Regional Jail after her bond was posted.
Urbana police said Castle wrote a message on the women’s bathroom wall at the high school that stated there would be an explosion at the school at a specific time.
The high school and junior high building was put in lockdown as authorities searched the campus.
Nothing was found during the search and police characterized the message as “a hoax.”
Castle is scheduled to appear in court Jan. 31 for a preliminary hearing.