MILFORD, Conn. (WTNH) – A Connecticut teenager charged in the 2014 death of a classmate has accepted a plea deal, News 8 reported.

18-year-old Christopher Plaskon has agreed to serve 25 years in prison for the stabbing death of 16-year-old Maren Sanchez.

Plaskon was charged with stabbing and killing Sanchez in the hallway of Jonathan Law High School in April of 2014. Plaskon was reportedly upset at Sanchez because she turned down his invitation to the prom.

Plaskon pleaded not guilty to murder. His attorneys have indicated they’re exploring an insanity defense.

Plaskon will be formally sentenced in June.