The student, whose identity was not released, brought the homemade explosive to Pine View High School in St. George inside of his backpack on Monday.
A student found a backpack with smoke coming out of it, police said.
The suspect was arrested after the school was evacuated and authorities, including the bomb squad and FBI, investigated.
A computer search indicated the youth had researched information about ISIS that was consistent with promoting the terrorist group. After a bomb-sniffing canine confirmed the explosive was real, bomb techs disarmed the device, which police said could have caused "significant injury or death".
Police believe the suspect, who is male but can not be named because of his age, is the same person who spray painted graffiti at Hurricane High School (18 miles away in Hurricane, Utah) in mid-February.
The backpack was left in the cafeteria of the high school during lunch where some students noticed it smoking. Police declined to identify the arrested teen or give his age.
No damage was reported and the school reopened Tuesday morning for classes. They alerted the resident school resource officer and faculty, which led to the school's 1,100 students being evacuated to the football field for the rest of the school day...
"By the end of (lunch), when the fourth period had started, then they evacuated us-which kind of anxious me because it took them the entire lunch period to evacuate all the students for something risky in the building", he said.
Washington County School district spokesperson Steve Dunham said Monday while the lack of injuries sustained in the particular incident was "a great outcome", the act itself was nerve-wracking.
The boy was arrested and booked into a detention center on charges of manufacture, possession, sale, use or attempted use of a weapon of mass destruction, St George police said in a statement.
"I think this is the first time anything like this has ever happened in our community", officer Ken Thompson with Hurricane City Police Department said.
Police delivered a search warrant at the home of the young male. Authorities initially thought the graffiti and flag at the school were pranks... Police found Hurricane High School was also graffitied in large text with the message "ISIS is comi [sic]".