It happened around 12:00 p.m. Thursday at the former Darley Road Elementary School located at 500 Darley Road.
Police say the woman, a secretary in the human resources department, was approached by the male suspect, forced into a bathroom at knifepoint, and assaulted.
The suspect then fled.
The victim told police it was the second time the man had been in the building on Thursday.
"The subject had been in the building earlier in the day, asking if they provided daycare," said Cpl. Jeffrey Hale of the Delaware State Police. "He then left, came back, and that's when the assault occurred."
The doors automatically lock, police said, and the man was likely buzzed in when he arrived the first time to ask about daycare. Investigators believe the man somehow rigged the door to remain open when he left, allowing him to get in the second time.
At the time of the attack, the victim was one of only two staffers in the building.
The suspect descrption is vague - the man is described as a black male, in his late 20's, medium build, medium height, wearing a white t-shirt and tan pants. He was also carrying white towel.
