A North Brunswick High School teacher and girl's soccer coach has been charged with sexually assaulting a 17-year-old female student, authorities said today. Matthew Engelson, 30, of Lawrenceville, was arrested after authorities learned he was exchanging sexually explicit instant messages with the girl and met her Saturday in South Brunswick, Middlesex County Prosecutor Bruce Kaplan said in a statement. Engelson has been a history teacher at the high school since September 2005 and was charged Thursday with official misconduct and sexual assault, both second-degree crimes. He has coached the girls soccer team since July. Investigators determined he began exchanging messages with the girl in October and also talked to her via cell phone, Kaplan said. Police also interviewed the girl and found other evidence to support the criminal charges, Kaplan said. Bail has not yet been set. North Brunswick school Superintendent Brian Zychowski posted a letter on the district Web site today stating that an instructor had been questioned by police. "This news is troubling and concerning," Zychowski said. He scheduled a meeting at 6 p.m. tonight in the high school.