The threat was found written on a girls restroom stall at the Clinton High School.
Students were taken to the junior high while police investigated and the students returned to class by 2:15 p.m. The threat turned out not to be credible.
There are security cameras outside the girl's bathroom, so investigators know who was coming and going.
They are interviewing students and say when they find out who did it, they will be prosecuted to the fullest extent, possibly expelled from school and possible criminal charges.