State police said school officials contacted them on Nov. 27 after obtaining a cellphone with sexually explicit text conversations between the victim and Ondish. She is also accused of buying the victim that cellphone, which is against school rules.
Ondish began working at the school, a rehabilitation facility for juvenile offenders, in June.
The victim told investigators he and Ondish began talking outside class, and their interaction progressed until they had sex in a classroom twice in November, investigators said.
"State police investigated this," said Trooper Dan Kesten. "It wasn't a relationship that the two were supposed to have; this was institutional sexual assault."
Police said Ondish admitted to the relationship. School officials had previously issued a warning to her about the victim, including having the victim in her classroom after hours.
Summit Academy released this statement Monday afternoon.
"The administration at Summit Academy was made aware of an inappropriate relationship between a teacher and a student and it was immediately reported to the state police. The teacher was then terminated."
Troopers said Ondish told them she was in an abusive relationship at home and developed an emotional attachment to the student.
She is charged with institutional sexual assault and faces a preliminary hearing in February.