Westminster Police Department spokeswoman Cheri Spottke said the boy didn't make any threats before starting the fire at about 7:15 a.m. at Standley Lake High School.
"There is no indication there were any threats to any schools," she said.
Spottke said a custodian was able to use a fire extinguisher to put out the blaze before it could spread. Several other students were in the cafeteria at the time, but none were injured.
She didn't know how the student set the fire, which caused extensive smoke in the building.
Jefferson County Public Schools spokeswoman Lynn Setzer said the school was closed for the day and students were sent home. She said she didn't know if students were in class when the fire was reported.
Setzer said students without cars were taken by buses to a local middle school, where they were picked up by their parents.
Security vehicles and police cruisers blocked the entrance to the school as light snow fell on a frigid morning.