Thirty-four-year-old Kenneth Martin was in court Tuesday when the judge set bond at $100,000.
Police say he had a long sexual relationship with one of those students, a 16-year-old girl.
Chicago Christian High School is a small private religious school. And police say it was in music class here where Martin began an inappropriate relationship with two female students.
"The relationship began, beginning with texts and phone calls and video chats, and then they began meeting in person," said Palos Heights Police Deputy Chief Dave Delaney.
Police say Martin, who went by Kenneth Whitney at the time, left the school after one year teaching there in 2008-09.
But his relationship with the two students allegedly continued for an extended period of time.
Martin is charged with one count of aggravated criminal sexual abuse against one alleged victim who was 16 at the time.
The school is on spring break this week. The superintendent issued the following statement:
"As soon as school officials became aware of this allegation against our former employee, the matter was reported to the authorities and we have been cooperating with the investigation. The individual has not been employed by our school since 2009."
Police say the relationship cannot by law be consensual when it involves a minor.
"In a position of trust and authority parents expect that from the schools that they have and I think they expect the police to act on that once the information is out," said Delaney.