The late 1920's and early 1930's saw many small independent districts on the outskirts of the city annexed into the school district. This caused an increase of students in the upper grades. In 1925, the School Board purchased the Lemp Triangle for $22,500. The Lemp Triangle was a triangular piece of ground where baseball was played before Boise Junior High was built in 1936. Baseball was still played after the school opened and a few of the neighbors weren't too happy with the baseballs coming their way. The foul balls came into their yards and the players kept trampling down their flowers and shrubs to retrieve the balls, so the neighbors plowed up the field. A backstop was set up and the field put back into shape and the games continued with no reprisals from the neighbors.
The junior high was built partially to facilitate the switch from an 8-4 school system to a 6-3-3 elementary/junior high/senior high system. Skeptics at the time said the new system would never work and that the Board and administration would regret the change to this new-fangled structure.

Upon construction of South Junior High in 1948, the school's name was changed from Boise Junior High to North Junior High.

There have been two additions to the school since it was constructed; a band room in 1949 and the cafeteria in 1968. There has been other remodeling done from time to time to enhance the library and other facilities. In 2008-09 the interior and the roof of the school gymnasium were demolished. The stage and exterior brick were left in place. New PE and athletic locker rooms were built in the basement of the gym, as well as a new weight room, classroom, wrestling room, PE/Athletic offices and storage rooms. The gym was enlarged by 40 feet with new restrooms and a new entrance. An elevator was also installed near the north interior entrance to the gym. The new gym was dedicated in the fall of 2009.