WARNING: Details in this story may be graphic for some readers

A substitute teacher at a suburban elementary school is accused of sexually assaulting a young female student.

Officials accused 56-year-old Joseph Gurzejk of inappropriately touching the 10-year-old girl after class at La Grange Highlands Elementary School.

Prosecutors allege Gurzejk molested the girl before lunchtime Monday after telling her to stay behind when the rest of the students left for lunch. They claim Gurzejk touched the girl's buttocks under her underwear while saying he was "getting nasty" and also touched her genital area over her pants.

The young girl told a classmate what had happened and then alerted school officials that same afternoon, prosecutors said.

Gurzejk, of Westchester, was charged Wednesday with predatory criminal sexual assault of a child and aggravated criminal sexual abuse, said Cook County Sheriff Thomas Dart.

According to prosecutors, Gurzejk admitted to the inappropriate touching along with looking at child porn.

The school said in a statement that it learned of the inappropriate conduct on Monday and immediately contacted the Department of Children and Family Services, police and the student's parents. Gurzejk, referred to as Mr. G, was immediately removed from the district’s subbing list following the allegations and was taken into custody Tuesday, authorities said.

Gurzejk has subbed in the district since 2004, school officials said.

“He cleared all required background checks, and no prior complaints have been reported,” the school said in an email to parents. “He has subbed in most, if not all, of the district’s classrooms.”

Gurzejk's bond was set at $750,000. He is next scheduled to appear in court on Feb. 24.