The first joint elementary and high school's first graduating class in 1890 comprised seven students—four girls and three boys.

[6] In 1910, Riverside High School's enrollment was approximately 500 students, and new facilities were required.

The earlier enrollment explosion waned as young men joined the armed forces.

1924–25 saw the Junior College and the Senior High School with growing enrollments, and so provided separate administrations for each.

[citation needed] During World War II, many Poly girls worked with a federal government–sponsored group called the High School Victory Corps.

[7] The girls helped make bandages and other needed items, or worked in essential industries after school.

Riverside Polytechnic High School is home to one of the original, still active Army Junior Reserve Officers' Training Corps (JROTC) units, established in 1917 and was originally called the Poly High Cadet Corps.