UNION GROVE -- A 15 year-old Union Grove High School student is under arrest in connection with a series of threatening messages that canceled classes today. The most recent threat was the fourth written warning of violence found at the school in the last 12 days. The first one was found on March 7th, and two others were found yesterday. "This is completely out of the ordinary," Union Grove High School Superintendent Dave Magar told TODAY'S TMJ4. "We have not experienced this before." The threats were found inside girls' bathrooms. At least two referred to someone having a gun and using it at school. Parents received a phone call, letting them know to keep their children at home Thursday. $500 reward was offered for credible information leading to an arrest. Anyone with any information is asked to call (262) 636-3323.