Police executed search warrants for the phone records of both the student, 18, and the teacher, who is 23. According to the warrant, the teacher allegedly performed a sex act on the student at least twice in late 2013 and provided him with test answers.
Authorities began their investigation after a high school resource officer interviewed a friend of the male student in late April after allegedly hearing of the situation from the school's principal, officials said.
The teacher has been interviewed by police as part of the investigation.
"We will continue to work closely with school officials and the State's Attorney's Office during the investigation," said Hampshire Police Chief Brian Thompson in a statement.
Text messages, phone calls, photos, videos and social media content are a main focus of the investigation.
School officials said this is the first year that the staff member involved has been with the district.
"A concern has been raised about inappropriate behavior that allegedly took place between an HHS staff member and a student," District 300 officials said in a message to parents Wednesday. "It is our responsibility to take such concerns seriously and to cooperate with the police as needed in their investigation."
No charges have been filed.