Christopher Serpentine, 33, of Brookhaven is charged with four counts of institutional sexual assault.
The school's website says Serpentine is a fine arts teacher who started in 2014.
According to the affidavit of probable cause, the Radnor Township Police Department says on May 23, a woman came into the station and told them she had an inappropriate relationship with Serpentine when she was a senior in 2017.
According to the complaint, she said she first met Serpentine when she was junior. In April of her senior year, she told police that Serpentine kissed her, then touched her.
She said there was additional contact, including on the day of her prom. A few days after she graduated in June 2017, she returned to the school, she added, where she and Serpentine had sex.
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Institutional sexual assault can be filed against a teacher, coach, counselor or anyone else in charge of teens, even if the accuser is over the age of 18.
According to the school, Serpentine had a clear criminal record before he was hired. He is currently on administrative leave without access to school grounds.
The school became aware of Serpentine's arrest following media requests Wednesday evening. A memo was distributed to all Carroll High families, that read, in part:
"Recently, school administration received notification through a third party that concerns regarding Mr. Serpentine's interactions with students had been reported. This notification instructed us to monitor his activity on school grounds. Out of an abundance of caution, we placed him on administrative leave and entirely restricted his access to campus. Last week, we were contacted by the Radnor Township Police Department in connection with this matter.
"We cooperated fully with the Radnor Township Police Department as it investigated Mr. Serpentine over the past week.
"We will take all possible steps to support our students and school families. We are encouraging our young men and women to speak with counselors should they need those services. In the coming days, we will monitor the situation at school to ensure that we are providing all possible modes of support."
Last month, another Carroll High employee, then-music teacher Jeremiah Triplett, was also arrested. The 30-year-old was accused of having illegal contact with a student and giving students marijuana edibles.