School officials said somebody broke into the school and stole the tests on Monday, CBS Denver reported. The suspect was described as a male juvenile, but officials said for legal reasons they wouldn't say whether the individual attends Cherry Creek High.
Authorities are trying to figure out how the teen got into the school just before midnight on Sunday, especially when it should be locked and has an alarm system.
About 800 juniors were scheduled to take the test Tuesday morning, CBS Denver reported.
"We were told they didn't have the ACTs," one student said.
"We worked really hard to try to study for it and they wanted to do it to get it over with and then it got cancelled," said another student.
"The area were the tests were stored was an isolated area of the basement and was securely locked. There were tools used to enter that door," said Tustin Amole with Cherry Creek Schools. "We have video surveillance of someone entering the building and then later leaving, carrying some objects in his hand."
Students who were supposed to have most of the day off because of the ACT test received phone calls saying Tuesday would be a regular school day.
The junior class tests have been rescheduled for the beginning of May, and students will likely be given a different version than the one that was stolen.
The Greenwood Village police are dealing with the crime as a theft case.