The 50-year-old custodian was walking from one building to another on the school campus at 301 E. Alaska Ave. around 9:50 p.m. when two men, one of them armed, approached him. Lt. Randall Boggs said.
Officials say the suspects ordered the man to lie on the ground and threatened to kill him if he didn't cooperate. Later they told the custodian to take them where the school's iPads were stored.
Once there, Boggs says they bound his hands with duct tape, bound the janitor to a desk and stole the iPads, valued at $350 each.
Officials say the uninjured custodian partially freed his hands and called police, and he was still partially taped to a desk when officers arrived.
Police were unable to locate the robbers.
According to Boggs, the robbery was the third in four months in which thieves targeted the school's iPads. The school was burglarized in September and October, but this is the first time the armed robbers accosted school staff.
The black male suspects are in their 20s with a medium build, police said.
One man is 5 feet 5 inches to 5 feet 7 inches and was wearing a gray hooded sweatshirt, blue jeans with a white design on the back pockets, white gloves and white and black shoes.
The other suspect is 5 feet 9 inches to 6 feet and was wearing a black hooded sweatshirt, a black leather jacket, dark jeans, gloves and black and white shoes.
(ABC7 News contributed to this report)