Officers say it happened inside a boy's bathroom at an elementary school in Sonora, which is east of Stockton.
Another student reported that two 8-year-olds and a 9-year-old child smoking pot last week.
This comes as a big surprise to other students and their parents.
"Shocked," one mother said. "To be in third grade and have their own pipe?"
A young student added, "It was bad because they were little kids and it shouldn't happen."
Police questioned the students, who were released to their parents.
The school district is not commenting.
It's not known what disciplinary action was taken. But students say they haven't seen the kids on campus since.