An Illinois teacher accused of sexually assaulting a 15-year-old student she tutored allegedly began messaging him using a digital school platform, which prosecutors say ramped up into flirtatious texts ... and a sexual encounter inside a classroom.

Christina Formella first got together with the student in 2023 when she was his freshman soccer coach at Downers Grove South High School ... this according to court documents, obtained by TMZ.

The docs say Formella started privately tutoring the alleged victim after he broke his collarbone. Prosecutors say Formella then gave him her number so they could play "8-Ball Pool" on their phones. The pair then allegedly exchanged provocative texts, followed by a FaceTime meeting between the student and Formella.

According to the docs, the teacher and her student began having feelings for each other, which came to a head one day in December 2023, when they met in a classroom at the school before classes began.  Prosecutors say Formella shut the door, and started kissing the victim, who reportedly fondled her, and she removed his clothes. They both allegedly took off her leggings, and he put on a condom and had sex with Formella. He was 15 and she was 28 at the time, prosecutors allege.

The docs say both later acknowledged to each other what they did was wrong, and they never had sex again, and spoke for the last time in February 2024.

Cops say the student's mom found the lurid text messages on her son's phone and reported them to the police department, which started the criminal investigation that led to Formella's arrest the next day, on March 16, 2025.