The Valley Courier reports the teen was taken into custody Tuesday after the threat appeared about Center High School in Center, about 20 miles north of Alamosa.
Center Consolidated School District Superintendent George Welsh says it appears the teen simply made an irresponsible remark and didn't plan to follow through on it.
Welsh says if an investigation indicates the threat was serious, the teen would be prosecuted.
Welsh notified community members about the incident, and numerous police officers were at the school the next day. No problems were reported, but at least 40 students stayed home from school on Wednesday.
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