Diagyan National High School was established on September 24, 2007 in Brgy. Diagyan, Dilasag, Aurora, in response to the challenges faced by students who had to travel about 30 minutes daily just to attend secondary school in the town proper.

The initiative was led by Avelino C. Silapan, then principal of Diagyan Elementary School; Domingo P. Moises, former barangay captain of Brgy. Diagyan; other barangay officials; and with the full support of then Dilasag Mayor Victorino B. Briones. A joint resolution from the barangays of Diagyan, Dilaguidi, and Dicabasan was passed to establish a high school in Brgy. Diagyan. This took place under the leadership of Schools Division Superintendent Antonio A. Gereno.

SDS Gereno coordinated with Mayor Briones for the construction of classrooms. While waiting for school buildings to be built, the Brgy. Diagyan Multipurpose Hall served as temporary classrooms when the school opened on September 24, 2007.

During its first year, the school offered First Year and Second Year levels with 159 students and five locally funded teachers. The pioneering teaching staff was headed by Ma. Elmie Pagaduan-Miague, followed a week later by Rodolfo O. Leti Jr., Julie Maryjane Verdadero-Dayao, Norma Tud-Abad, and Beverlin Gonzales-Friginal. The school was initially supervised by Avelino C. Silapan until Martes S. Rivera assumed leadership in November of the same year.

Originally named Dilasag National High School Annex, it was later renamed Diagyan High School, and in 2015 officially became Diagyan National High School.

The school stands on a two-hectare lot donated by PATECO, a logging company based in Brgy. Diagyan. It is located near the barangay plaza along the provincial road leading to Dinapigue, Isabela, and serves students from Brgy. Diagyan, Brgy. Dilaguidi, and even some from Dinapigue, Isabela.

At present, Diagyan National High School offers both Junior High School and Senior High School programs. The Senior High School includes the Humanities and Social Sciences (HUMSS) strand under the Academic Track and Agri-Fishery Arts under the TVL Track. The school now has 18 nationally funded teachers, a school principal, an administrative assistant, an administrative officer, an administrative aide, and two utility personnel under contract of service.

The campus consists of nine school buildings housing classrooms for various grade levels, a library, guidance office, ICT rooms, and TVL workshop areas, as well as a covered court for sports and school events.