Police said the man got into Downers Grove North High School through an unlocked window early Wednesday.
The suspect demanded money from a maintenance worker, police said. A second employee came in, and that's when police say the man ran off.
Police said the offender was "approximately 50-60 years of age, 5'6" tall, with gray facial hair" and "wearing a black puffy coat, a dark baseball cap, green pants, tan boots, tan gloves, and a black drawstring bag."
No further information was available as Downers Grove police continue to investigate.