Police haven't released the suspect's name, but people who saw him taken into custody identified him as Chris Plaskon, a friend of the victim, 16-year-old Maren Sanchez.
Plaskon's attorney, Richard Meehan, says his client is being held in a hospital under psychiatric evaluation and will not appear at his arraignment, scheduled for Monday in New Haven.
Plaskon is charged as a juvenile but Meehan has said he expects him to eventually be charged as an adult.
The stabbing occurred Friday morning at Jonathan Law High School in Milford, hours before the school's prom, and authorities are investigating whether Sanchez was stabbed after turning down the boy's invitation to the dance.
Connecticut's chief medical examiner said Sunday that Sanchez died of wounds to her torso and neck.
Milford Alderwoman Greta Stanford said the school would remain closed Monday. A memorial service is scheduled for 7 p.m. on the school's football field. Organizers are seeking donations so the junior class can buy a memorial bench in Sanchez's honor.
Mark Robinson, a technical education teacher who saw the suspect being taken out of the school in handcuffs, said Plaskon is the third of five brothers and has a good sense of humor. His family has deep roots in the community, Robinson said.
"There's no reason to suspect he would have done this. I think that's what makes it harder," Robinson said.
Classmate Imani Langston, who saw Plaskon being read his rights and taken away in a police car, said Sanchez and the boy were just friends and had never dated.
Sanchez, a member of the National Honor Society who was active in drama and other school activities, had been focused on prom in the days before her death. She had posted a photograph on Facebook of her blue prom dress and was looking forward to attending with a new boyfriend.
People who know Plaskon said he is an athlete and described him as genial and respectful.
"His family is devastated not only for him, but the youngster who was killed," Meehan said." It's a terrible situation all the way around."
