Roy High School student Dallin Morgan entered the plea Thursday to criminal mischief. He'd originally been charged with felony possession of a weapon of mass destruction.
Morgan and a 16-year-old boy were arrested Jan. 25, after other students reported getting disturbing text messages from the younger suspect.
Police recovered no bomb, but found plans for an explosive and maps of the school and its security system.
The 16-year-old pleaded guilty to possession of a weapon of mass destruction and was ordered to at least six months in juvenile detention.
The Associated Press isn't naming him because he's a minor.
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