The arsenal included military uniforms and armored vehicles as well as thousands of rounds of ammunition, the BBC reported.
Police were alerted after students told school authorities they saw the gun in the bag of a 9-year-old classmate. The gun, which the report said was loaded, was confiscated.
Police raided the boy's home in Hermosillo, where the arsenal was found.
Among the weapons were an AR-15, 13,000 rounds of ammunition, pistols and assault rifles, police said.
Bulletproof vests and two money-counting machines also were found.
A woman thought to be the boy's mother was arrested at the scene but a man managed to escape.