Surveillance video shows the men, with their faces partially covered, roaming the halls of Fairmont Elementary at about 4 a.m. on Saturday.
According to Deer Park ISD, most doors were locked, but the men entered a music room and took beginner instruments worth $200.
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The district said the suspects also gained access to the roof and sprayed a fire extinguisher inside the building.
"This is something that happens on a day-to-day basis. I don't think that anybody's surprised that the door was left unlocked," said Michael Matranga of the M6 Global Defense, a school security consulting firm.
While no students were present during Saturday's burglary, Matranga said questions still need to be addressed.
"We need to go back and look at when the last inspection was, what the condition of the door was, and was it mechanical or caused by human error?" he said.
"In all likelihood, an employee used the door, and it simply didn't close and lock properly," Deer Park ISD wrote in a statement to Eyewitness News, citing the building's 34-year-old age.
"We are taking this opportunity to remind employees at all of our schools to double-check all exterior doors when entering or exiting the school building," the district wrote.
According to Fairmont Elementary, the suspects were inside the school for 23 minutes but on school grounds for an hour and 15 minutes.
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"Why was the alarm not active and operational? Why was there not a notification to authorities that there was a breach of that door or movement on that campus? Matranga said.
The district said Fairmont Elementary has passed all intruder inspections in the state inspection program's two-and-a-half-year history.
If you recognize the men in the surveillance video, contact Pasadena police at 713-475-7892.
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