[3] Eleventh and twelfth grades were soon added, and in June 1911 the first students to complete four years of high school graduated.

[9] The school grew in size, with the 1920 graduating class totally 36 students, the largest number up to that time.

[11] Located near downtown Hillsboro, at NE Sixth Avenue and Lincoln Street, the site became a junior high once the current campus was built beginning in 1969.

[11] Students complained that the new layout that consisted of classrooms spread out over the campus instead of a single central building was a poor choice for Oregon’s rainy and cold winters.

After the expansion Hilhi had a total of 256,652 square feet (23,843.8 m2) of space spread out among eleven single-story buildings on campus.

[17] In the fall of 2015, the school became the first in the district to implement a mariachi band program into the curriculum, started by Hilhi alumni and Choir Director, Benjamin Noyes.

[31] Mouse Davis, an early proponent of the run and shoot offense, coached Hillsboro High to the 1973 football state championship.

[34][35] The school previously held a homecoming king and queen ceremony, but this was terminated by the student government in 2016.

[43] Beth Graser, the chief of communications of the school district, stated that the move was made to prevent a popularity contest from occurring, while the class president of the 12th grade and the student government adviser stated it was done to make student culture more inclusive for genderqueer individuals.