A primary and middle school teacher allegedly bound, gagged and fondled 20 female students, some as young as nine, acting out bondage fantasies from pornographic videos while colleagues twice failed to alert the authorities to complaints against him.
The allegations have shocked the Chicago suburb of Berwyn, where Robert Sperlik, 45, was a teacher with 18 years' experience.
He faces 219 charges of child sex abuse, including multiple counts of criminal sex abuse, indecent solicitation of a child, aggravated kidnapping and unlawful restraint.
Authorities say he used insulating tape and rope to tie students to chairs before molesting them. At least once, he inserted a rag into a student's mouth then taped over it, said prosecutors. Berwyn's public safety director Frank Marzullo said some victims were enlisted to tie up their classmates.
Police found more than 30 adult bondage videos at Mr Sperlik's home and Polaroid photographs of some of the victims. The masking tape made identification difficult, said Mr Marzullo.
Mr Sperlik allegedly abused the female students - then aged nine to 14 - at six Berwyn schools between 1998 and 2003. He denies all the charges. Last week, the head at one of the schools was arrested and charged with failing to report allegations against Mr Sperlik in 2001 of "inappropriate sexual conduct with two girls" and "inappropriate touching".
"Most parents are outraged that this (went) on for a period of time," said Mr Marzullo. "It appears school officials did not take the next step."
Mr Sperlik was still teaching until late January, when he was arrested after the parents of a 15-year-old girl complained to police. Since then, other alleged victims have come forward.
Karen Grindle, principal of Pershing school, has denied that she was negligent in failing to report the 2001 allegations against Mr Sperlik to the local education authority. She and the authority declined to comment on the allegations. Last month, officials at Berwyn School District 100 said they had begun an internal inquiry into why staff had not acted on the allegations earlier.
Mr Marzullo said the alleged abuse escalated from what was initially a "game, a fun type of activity" and that Mr Sperlik "was a well-liked teacher".
Mr Sperlik's lawyer, William Hedrick, has said that police disclosures about the case have jeopardised his client's prospects of receiving a fair trial.