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 opened PNHS- 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 batch 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 the 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 was: 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 graduate 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 the 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 levels, four (4) sections each in the third year and fourth year levels. That school year produced 123 graduates. PNHS-Don Galo Annex had a joint graduation with PNHS-La Huerta held at San Dionisio Gym.