PNHS-Don Galo Annex had come to existence when there was over pouring of students in the main high school, and particularly PNHS-La Huerta Annex. To solve the problem, Dr. Rolando Magno, Schools Division Superintendent opens the Don Galo Annex. This annex has been housed at the Don Galo Elementary School Campus since June 21, 1999. Dr. Rolet M. Palacio was designated as the first Officer-in-Charge. The first batches of teachers were Ms. Nerissa L. Lombos, Mrs. Olivia L. Caturay, Ms. Feligarda Etac, Mrs. Gina Tindoy and Mr. Joel M. Merjudio.

The Annex started to function with six (6) classrooms; one (1) for the faculty room and two (2) in the fourth year, under and one (1) for the administrative office. There were 284 enrollees with three (3) sections each for the first year and second year levels. The school operated with one session.

In school year 2000-2001, under Mrs. Teresita L. Flores as Officer-in-Charge, the student population increased to five hundred ninety five (595). There were four (4) sections in the first year level, three (3) sections in the second year and two (2) in the third year. This time classes were conducted in two shifts.

In 2001-2002, the main school recalled Mrs. Teresita L. Flores, Dr. Gregorio T. Capiral assumed office as Officer-in-Charge with a population of 989 and offered complete year levels. The distribution of sections were: six (6) for the first year, four (4) sections in the second year, three (3) sections in the third year and two (2) in the fourth year. That year, yielded the first graduated of Don Galo Annex with a total of 110 students.

Another achievement that year was the birth of a school organ, the “Scroll” under the advisory and coaching of Mr. Alejandro Oliva of the English Department.

In school year 2002-2003, Mrs. Teresita L. Flores as Officer-in-Charge, Don Galo Elementary School in favor of the Secondary School, initiated by Dr. Rolando L. Magno gave up another six (6) room building (Bagong Lipunan). The additional rooms answered the unprecedented increase in enrollment. This time, the enrollment reached 1308 with six (6) sections in each in the first year and second year level, four (4) sections each in the third year and fourth year levels. That school year produced 123 graduates. PNHS-Don Galo Annex had joint graduation with PNHS-La Huerta held at San Dionisio Gym.

In 2003-2004, the population increased to 1450. This time, we had seven (7) sections in the first year, six (6) sections in the second year, five (5) in the third year, and four (4) sections in the fourth year level.

Improvement of physical facilities could be observed. Through the linkage with the Rotary Club of Parañaque-Metro South, a library was established and has been functional since January 3, 2003.

In 2004-2005, the school had to reduce the number of sections in the first year level from seven (7) to six (6) sections to answer the increased number of the fourth year students, thus the fourth year level has five (5) sections.

In school year 2005-2006, Mrs. Teresita L. Flores retired and a new Officer-in-Charge was assigned in the name of Miss Estrellita C. Arceo. The school increased the number of sections in the first year from six (6) sections to seven (7) sections to answer the number of first year students, thus, the second year had four (4) sections, third year level had increased from four (4) sections to five (5) while the fourth year level remained as five (5) sections.

In 2006-2007, the school had increased the number of sections for the second year from four (4) sections to six (6) sections to accommodate larger enrollment for the second year, hence reducing the sections in the third year level to four (4) sections, the fourth year still on five (5) sections.

PNHS-Don Galo Annex has surpassed all odds in spite of meager resources. Strengthened by the spirit of vocation and inspired by the students’ needs, the staff has been doing their best to answer the students’ quest for knowledge.

For school year 2007-2008, the school had an enrollment of 1449.This marked Don Galo’s participation to the Red Cross First Aid Competition, a national competition taking pace every four, (4) years. The school was able to win the following championships-Division, Regional Champion and 1st Runner Up in the National Level held at the Rizal Stadium.

The school also bagged the 2nd place, twice in a row in the “SabayangBigkasan 2007”. The students of the Don Galo also did their best and landed 2nd Runner Up in the Men’s and Women’s Volleyball sponsored by the PCAA and 3rd in Basketball. The school is also proud to have produced an outstanding student for the Parañaque Youth Shine Awards for Health in the person of Jeffrey Morales.

School facilities such as girls’ comfort rooms, guidance center/records office; separation of power lines from the elementary level was also enhanced.

PNHS-Don Galo Annex ranked third among the secondary public schools in the Division of Parañaque in the 2007-2008 National Achievement Test (NAT) for the second year students. Although the MPS of the School is below mastery level, the staffs is in cognizant of their task to improve the quality of instruction in order to raise the academic performance of the school.

For school year 2008-2009, the school’s population increased to 1701. The school maintained six (6) sections in the first and second year levels and five sections in the third and fourth year levels.

In September 1, 2008, a new Officer-in-Charge was designated Mrs. Leonora J. Nofuente, the Science Department Head of PNHS-Main.

School improvement projects such as rehabilitation of the comfort rooms for boys and girls, installation of drainage pipes, installation of roof gutters in the third floor Building A, concreting of the path walk and landscaping of the garden and providing garden benches gave new look to the school.

For school year 2009-2010, the school’s population decreased from 1701 to 1606 to reduce the class size to a manageable level. The school still maintained six (6) sections in the first year and second year levels and five (5) sections in the third year and fourth year levels.

On June 7, 2010 of school year 2010-2011, Mr. Rafael Centina assumed office as the new Officer – in - Charge. The school’s population increased from 1701 to 1758. The school maintained six (6) sections in the second year level. For first year level it was increased to seven (7) sections due to large population. The school borrowed 2 rooms from Don Galo Elementary School. Third and fourth year levels increased from five (5) to six (6) sections because several students transferred from private schools to PNHS-Don Galo Annex due to a very high tuition fees.

During this time a two (2) storey building was constructed as well the covered waiting area and pathway.

On March of the same school year, Mr. Recto Makinika was designated as the new OIC. From May 1 – 30 of 2011, Mrs. Zolaida Bonacua, Education Supervisor 1 for Science was the Officer – in – Charge of PNHS – Don Galo Annex.

For school year 2011-2012, the school population increased from 1758 to 1869. The school increased the number of sections in the first and second year levels to eight (8) sections to answer the number of students. The school maintained six (6) sections in third and fourth year levels. Apart from the regular high school, Project EASE (Effective Alternative Secondary Education) opened for all year levels accommodate students who are at risk of dropping out.

In June 1, 2011, a new Officer-in-Charge was assigned in the name of Mrs. Wilma T. Loria. Then there was a change of Officer-in-Charge in PNHS – Don Galo Annex in the person of Mr. Carlito T. Adducul on February 1, 2012.

For school year 2012-2013, as of July 31 2012, it has a school population of 1695. The number of classes are maintained for the second year and fourth year levels. The third year level has eight (8) classes and six (6) classes for Grade 7 which is on its first year of implementation. The Open High School Program (OHSP) provided additional venue for students who are at risk of dropping out to finish secondary education. It has one (1) class for each year level and replaced Project EASE classes of the previous school year.

Mr. Adducul sought and planned for school’s improvement which was followed through by the next OIC, Dr. Marilyn B. Timtiman. Dr. Timtiman took office July 2, 2012, and on the same month, the new two-storey four-classroom Olivarez Building was blessed and turned-over to the school. Though she took a short leave of absence from work for health reason, her duties and responsibilities were carried out smoothly and effectively because of the gracious supervision and administration of Mrs. Antonia Capiral, Education Supervisor for Values Education and Special Education, the new OIC- Principal who assumed office on September 4, 2012.

When Dr. Timtiman resumed office on September 25, 2012, the library is restored and is operational. She has more projects to initiated and implemented. For the school year 2013 -2014, the school is gearing up to achieving its targets. The enrolment rate increased, opening of one-shift class and revitalized administration, teaching and non-teaching staff.

Mrs. Marilou A. Apostol, a young and dynamic educator – administrator, took seat as OIC – Principal on July 1. 2013. She initiated changes that foster the school to be progressive.

Ms. Veronica Francisco, when assumed office as the OIC-Principal on April 1, 2014 had a smooth turn over of duties and responsibilities. Eventually, through her leadership, the school's performance improved towards achieving its mission and vision. Her term lasted until June, 2018.

Further, Ms. Wilma T. Loria took over as the next school principal on July, 2018 However, she had a short term of leadership as it lasted to July 2019. During her term, there were many significant winnings of the school treated as accomplishments.

Currently, Mr. Nestor T. Torrecampo is the new OIC-Principal. He assumed office on August, 2019. He continues the best practices which were started by the former school heads and still introduces best approaches towards leading a school aiming for success. He is naturally an efficient leader where he analyzes better strategies for administration and supervision gearing to high performance.