The suspect was booked as a juvenile on a class 6 felony of disrupting a school.
Other students saw the threat, and an adult reported it to police.
Police have not revealed the name of the student. Here is the message the school sent to parents:
Hello, this is Jon Lansa, principal of Amphitheater High School. I wanted to update you on a situation we were made aware of early this morning. A parent called police after seeing a threatening Instagram post made by a student. We have worked in cooperation with the Tucson Police Department to ensure the safety of our campus and the student has been arrested. The student was not on campus at any time
We are disappointed that this incident happened on our first day of school, but want to assure you that we take all threats seriously and we will always take the proper precautions to ensure our school is safe and secure. If you have any questions, please call me directly at 520-696-5372.
According to Amphitheater School District spokesman, the student will receive a mandatory long-term suspension and be expelled from the school.