Lana Trotter, 27, of Summit House in West Chester, surrendered on Thursday morning.
Trotter had been the softball coach at Delaware County Christian School in Newtown Square.
Police say she started a sexual relationship with a 16-year-old player in January of 2012. They say the relationship lasted until the girl broke it off at the end of the softball season last May.
"This is outrageous conduct because of the nature of the relationship between the coach and the child," said Jack Whelan, Delaware County District Attorney.
Investigators say they were alerted in July when the girl's parents found sexually explicit text messages on a cell phone she had been given by Trotter.
"They were very alarmed as to what they read and discovered a relationship between the coach and the child," said Whelan.
Trotter was a pitcher for the softball team when she attended DCCS. She also played in college and returned to DCCS as head coach in 2010.
Action News tried to talk with the school administrator.
"At this point and time, I need to ask you to take your camera and the truck off of campus," said Timothy Wiens, DCCS Head of School.
The school posted a message for parents on its website.
It doesn't name Trotter or her position and refers to her only as a former part time employee.
The investigation remains open, as police try to determine if there are any more victims.
Trotter turned herself in Thursday morning to be arraigned on two serious charges.
"She's being charged with two serious felonies - institutional sexual assault and corruption of the morals of a minor," said Whelan.
The District Attorney says prosecutors will be seeking serious jail time for Trotter. She was taken into custody but released after posting 10 percent of $50,000 bail.