An eighth-grade teacher at Slippery Rock Middle School is facing two felony charges after allegedly touching four girls inappropriately during his math class.

State police on Thursday filed charges of institutional sexual assault and corruption of minors against Robert P. Willison, 51, of Slippery Rock, after an investigation that was touched off by a complaint filed through ChildLine, the state's avenue for people to report suspected child abuse.

Police say Willison first touched one of the four girls on Sept. 20 during class, after she asked him to help her with homework. Police said the girl reported that Willison got down on his knees near her desk and put his hand on her chair. He then allegedly “swirled and rubbed his thumb” on her rear end for “some time,” according to court documents.

Police say they interviewed a student who sits next to the girl in Willison's class. That student told troopers they had seen Willison touch the girl, and reported that the victim also told them about it after class.

In court documents troopers allege that two 13-year-old girls also reported having similar encounters with Willison in the weeks before the Sept. 20 incident.

In a fourth encounter with another middle schooler, police said, a 14-year-old girl told troopers that Willison also touched her rear end last year, while she was walking out of his classroom.

According to the office of District Judge Bill O'Donnell, Willison has made arrangements to turn himself in to police on Monday. It was not immediately clear if he had an attorney and a call Thursday to a number for Willison was not answered. He did not return a message left seeking comment on the allegations.

According to police, Willison has previously been suspended by the district for inappropriate conduct while at school.

In 2012 he was suspended for “inappropriately touching a female teacher's butt in the copy room,” according to court documents. Police said the incident was seen by another teacher who confirmed it to troopers during their investigation.

In 2015, police said, Willison was again suspended for touching another teacher and making inappropriate comments about students. According to court documents, that teacher told troopers that Willison grabbed her rear end with both hands during a track practice and also made suggestive comments about the chests of middle school girls who were stretching during practice. She also told police that she saw Willison “touch and graze” a middle school girl who was performing a track exercise.

Police said the incidents were all reported to the district.

Superintendent Alfonso Angelucci said Thursday that Willison has been suspended “since the allegations were made.”