A teacher at Archbishop Carroll High School in Radnor struck up a friendship with a female student in 2017, police said Wednesday, and the relationship turned sexual while the teen was still enrolled in the school.

Christopher Serpentine, 33, was charged with four counts of institutional sexual assault, according to Radnor Township police. He was freed after paying 10 percent of his $100,000 bail, and has a preliminary hearing scheduled for June 27.

Serpentine is the second teacher at the school in as many months to face criminal charges of this nature. Jeremiah Triplett, 30, was arrested May 1 and accused of kissing, groping and exposing himself to at least one student he had supplied with alcohol and marijuana-laced brownies.

Serpentine, who is listed as a member of the fine arts department on the school’s website, could not be reached for comment.

In a letter sent to parents on Wednesday, school administrators said an unidentified third party alerted them recently to “monitor [Serpentine’s] activity on school grounds.”

Serpentine was placed on administrative leave as a result and was barred from the campus, the letter said. During pre-employment screening, Serpentine passed all child-abuse clearances and had a clean criminal record, according to administrators.

Investigators from Radnor Township contacted school officials about the pending investigation, and officials said they are cooperating with law enforcement.

The now-former student told Radnor police on May 23 that her relationship with Serpentine began during her senior year, when he served as the school’s director of communications. It was unclear from court paperwork how old she was at the time.

The two became friendly through a smartphone-based trivia game, the woman said. Through the game, she said, they discussed “personal issues,” according to the affidavit. She told police that she “felt thankful that she finally found someone she could trust" and that Serpentine was “the only person who understood her.”

She said they began a physical relationship, having sexual contact multiple times both on and off school property between April and June 2017, when she graduated from Carroll. After her graduation, she told police, they had sex for the first time, and continued to do so the following summer, the affidavit said.