The alleged victim is a 17-year-old student at Carl Sandburg High School in Orland Park.
Steven Reynolds is being held on $250,000 bond. He is charged with sexually abusing the female student.
Investigators say the alleged abuse did not happen on school grounds, and they believe there is one victim.
Reynolds is charged with two counts of aggravated criminal sexual abuse, a class 2 felony. The Dupage County Sheriff is describing the alleged incident as "inappropriate sexual contact."
Reynolds is a popular figure at Carl Sandburg, and students Tuesday were having a hard time coming to terms with the news.
"It's horrible, actually," said parent Julie Casica. "They have the power, and sometimes the kids feel trapped, I think."
Casica has two sons at Sandburg High School. And, still, she says school officials have yet to inform parents about the Reynolds' arrest.
Reynolds is a substitute teacher and freshman basketball coach.
"It's kind of scary too. When we live in this area, we feel our children are safe. But it happens everywhere," said parent Laurie Heemstra.
According to the DuPage County Sheriff's office, the alleged incident happened at Reynolds' home in Hinsdale on January 28. A press release issued earlier says the incident "came to light when the victim disclosed the alleged contact to her friends who subsequently informed school officials."
No one at Carl Sandburg or at the school district commented publicly Tuesday, but in a written statement, they said they only got the information Monday: "The administration immediately notified DCFS and local police and continues to cooperate fully with authorities in this ongoing investigation."
As school let out Tuesday, ABC 7 spoke to some who know Reynolds. They say no official announcements were made, but students were speaking of little else Tuesday.
"He subs for classes. He was a nice guy, I don't know, I would have never expected it," said Michelle Heemstra, a junior at Carl Sandburg High School.
"He's very well liked. He's more than a coach, he's a friend," said Sandburg junior Jimmy Hoye. "I would say that it's shocking to hear."
In addition to his work at Sandburg, Reynolds is also a youth leader at Hope Covenant Church in Orland Park. The pastor declined to say whether Reynolds would be allowed to keep his position.