With seventy-four students in the first four grades, Holy Trinity School opened on September 12, 1949. The originalschool was housed in the parish hall which had been partitioned to form four classrooms and a small hall at the back ofthe building. A new two story, eight room structure replaced the smaller school building in 1965.

Holy Trinity School has been staffed by two orders of sisters. The Sisters of the Immaculate Heart directed educationfrom 1949 to 1968. Felician Sisters replaced the Immaculate Heart Order from 1968 to 1992, after which the ImmaculateHeart Sisters returned from 1992 to 1995.

In spring, 2008, the decision was made to close St. Casimir School at 2714 St. George St., Los Angeles, California, due to declining enrollment. At that time The Department of Catholic Schools of the Archdiocese of Los Angeles invited the pastor and principal of Holy Trinity School to determine the feasibility of adding the St. Casimir campus to Holy Trinity School to accommodate any students affected by that school’s closure and to permit program expansion for Holy TrinitySchool at both campuses. This plan, approved by the Archdiocese of Los Angeles, was enacted beginning August 2008.The operation of the two campuses, “The Academy” for Kindergarten through grade five and “The Prep” for grades six through eight, provided additional space for both programs. In 2013 a Transitional Kindergarten program was added at The Academy and 5^th grade was moved to The Prep.  The addition of the Academy campus provided the school with a kitchen enabling a daily hot lunch program for both campuses, and an auditorium. The close proximityof the two sites (1.4 miles) and the staggered start and end times at the two campuses facilitated the drop-off and pick-uptimes for parents with children on both campuses. 

A new parish administrator was appointed in July 2014, and a new principal took on leadership of the school in November 2014.   With enrollment at 140 students for 2014-2015, and pastoral concerns of ministering effectively to families on both campuses, the pastor and principal began the process of evaluating the success of the two campus model.  After extensive consultation with the families of the community, the Department of Catholic Schools, and the Regional Bishop’s office, the decision was made to have Holy Trinity return to its original one-campus model.  With the original campus designed for eight grades, it was necessary to do some creative planning and reconfiguration to house all ten homerooms. The campus move was completed over Easter break in the 2015 – 2016 school year, allowing families to complete the year on one campus. 

In 2017, Holy Trinity School was the recipient of several grants.  First, a special grant from Shea Family Charities allowed the school to reconfigure the existing eight classroom floorplan to create two additional classrooms and a faculty lounge.  Additionally, all of the rooms were freshly painted, received new LED light fixtures, new window treatments, and new magnetic white boards and tack boards, with mounted digital projectors.   Many of the classrooms also received new tile flooring as well as new cabinets.  The front office was also reconfigured to create a more open and welcoming look.  New drinking fountains with a modern filtration system were also put into place.  Additionally, the school received a special grant through the Archdiocesan C3 program (Catholic Communication Collaboration) and the federal E-Rate program.  Working in conjunction with the renovation project, they installed state of the art wireless connectivity and networking throughout the school.  Yet another major blessing came through both Shea Family Charities and the C3 program.  The school received iPad pros for all of the teachers, and generation five iPads for all of the students, making it possible for the school to begin a 1:1 blended learning program in the fall of 2017.  Additionally, the teachers received laptops for school use, and the middle school received 56 new Chromebooks and two charging carts for student use. 

In spring 2020, following the directives of the Archdiocesan, state, and federal authorities to respond to the COVID-19 pandemic, the school moved to an all Distance Learning platform.  Students were issued their devices (Chromebooks and iPads) for home use and instruction was provided to all students via Zoom and Google Classroom.  In fall of 2020 the school opened in a Hybrid Flex model with some students on campus and others served through Distance Learning.  That model was maintained all year, with the vast majority of students returning to safe in-person instruction after Easter Break.  Full in-person learning returned in the 2021-2022 school year. 

With a growing enrollment of 120 students, a ten teacher lay faculty, and a dedicated administration, the community eagerly anticipates the launching of a new school year. 

The school is fully accredited by WCEA / WASC through June 2025.