St. Pius X - St. Matthias Academy is a coeducational private, Catholic high school for grades 9–12 located in Downey, California.

James W. Keefe was named principal in 1967, and the following year, instituted a concept of modern teaching, the Model School Project.

Pius employed an educational model called "non-gradedness," which placed a student in a class where his or her present achievement was the deciding factor.

[3] Declining enrollment from the 1980s onward placed financial pressure on the Archdiocese's high schools, prompting Pius X to begin fundraisers and to form a new alumni association in 1986.

It was decided to convert the facility back to a co-ed school, and the name St. Pius X - St. Matthias Academy was chosen to honor the traditions of both institutions.

St. Pius X-St. Matthias Academy or PMA as it is known among the community, serves a co-ed population of approximately 400 students from diverse economic and cultural backgrounds.

St. Pius X - St. Matthias Academy competes with other southeastern Los Angeles County Catholic schools in the Del Rey League, in the CIF Southern Section.