A teenager from Milford, Connecticut, has been sentenced to 25 years in prison for murder in the stabbing death of a classmate at their high school hours before junior prom in 2014.

Christopher Plaskon, 19, pleaded no contest to murder in March in the killing of 16-year-old Maren Sanchez.

Sanchez was stabbed to death in a stairwell at Jonathan Law High School in Milford on April 25, 2014. Plaskon's family and friends have said he was upset Sanchez turned down his prom invitation.

A judge sentenced Plaskon to 25 years in prison Monday morning, calling the sentence "appropriate, just and fair."

Plaskon will be eligible for parole in 13 years, after 30 percent of his sentence, because he was a minor at the time and has served two years.

Sanchez's mother, Donna, made emotional statements in court before the sentencing.

"She was my partner in crime, my best friend, my greatest joy," Donna Sanchez said.

Sanchez's father called the sentence too lenient.

Before the sentencing, Plaskon's attorney apologized to the Sanchez family on behalf of his client and his client's family.

Defense attorneys have said he showed signs of psychosis and they considered an insanity defense.

Officials from the state's attorneys office said the sentence issued was the best they could hope for based on the facts of the case, the defendant's age and changes in state law on sentencing juveniles in serious cases.

Sanchez's parents have filed a lawsuit, accusing the school system of negligence, saying she told a guidance counselor that Plaskon was troubled and capable of hurting others.

School officials have declined to comment.