MILFORD, Conn. — A teenager was sentenced on Monday to 25 years in prison for fatally stabbing a classmate after she turned down his invitation to the junior prom.

A judge sentenced the teenager, Christopher Plaskon, in Superior Court here. Mr. Plaskon, 19, pleaded no contest to murder in March in the killing of Maren Sanchez, 16, inside Jonathan Law High School in Milford on April 25, 2014, the day of the prom.

Ms. Sanchez’s parents said after the hearing that they were frustrated that the prison sentence was not longer. Mr. Plaskon will be eligible for parole in 13 years under new state juvenile sentencing laws.

“The message that it sends is that if you commit a crime as heinous as murder in the state of Connecticut, you’re going to do very little time, you’re going to walk out with a college degree and you have your life back at one point,” Ms. Sanchez’s father, Jose Sanchez, said. “And it’s not the message that we want to send to the children in our schools when this has become an epidemic throughout the country.”