It happened on Wednesday at about 1:35 a.m. at the high school at 4436 Main Street, police said.
Police released photos of a suspect they say entered the school through an unlocked window.
The suspect allegedly approached a maintenance worker and demanded money.
He then approached a second worker and then ran away.
Police said he was last seen traveling southbound on Forest Avenue on foot.
The suspect did not have a weapon.
Anybody with information is urged to call police at 630-434-5600.