A teen boy slashed a 16-year-old girl to death at their Connecticut high school Friday morning — after he asked her to be his date for the junior prom and she turned him down.

The girl, Maren Sanchez, had planned to go to the prom Friday night with another boy, a friend of her alleged attacker.

“Yay, so excited!!!!!” she had written under a recent Facebook photo of herself looking beautiful in her teal prom gown.

Sanchez’s friends recounted the morning encounter and grisly attack.

“He asked her to the prom right there, and she rejected him,” Imani Langston, a junior who described herself as Sanchez’s best friend, told The Post.

Another junior, Sam Garcia, 16, recalled: “There was blood on her neck. It was awful.”

Sanchez was stabbed multiple times with a kitchen knife, including in the face and chest, as she struggled with the spurned boy near a stairwell at Jonathan Law HS in Milford, officials said.

The popular girl, who had wanted to go into music and acting and was on multiple sports teams, fought back, even biting the boy as he strangled and cut her, sources said.

But the boy — identified by multiple classmates and their parents as “class clown” Chris Plaskon — overcame her and threw her down a flight of stairs.

Plaskon immediately allowed himself to be handcuffed by a school security guard and has been charged with murder.

He was being held over the weekend on suicide watch at an adolescent psychiatric ward, sources said.

The boy and girl had been platonic pals, their classmates said. He had secretly carried a torch for her, but she favored another boy — their mutual friend for years, school hockey star Jarrod Butts, their friends said.

“Chris has had a crush on Maren for as long as I could remember,” said ­junior Mick Murray, a friend of Plaskon and Butts. “All three of them grew up together.”

Plaskon was angry when Sanchez began dating Butts several weeks ago, multiple friends said.

It was Butts with whom Sanchez had agreed to go to the prom — and Plaskon knew that, pals said.

Butts expressed his grief over Twitter on Friday, tweeting, “I miss you.”

I miss you — Jarrod Butts (@jarrod_butts) April 25, 2014

Sanchez had posted her prom-gown selfie in early March on a Facebook page she and her classmates set up to compare dresses.

“The last day I saw her, she was showing me her prom dress. She was so excited,” said a girlfriend, Megan Gossett.

Her family released a somber statement Friday night about their sudden loss.

“The unexpected and sudden loss of our beloved family member Maren Victoria Sanchez is both shocking and devastating,” said her cousin, Edward Kovac.

“Maren should be celebrating at her prom this evening with her friends and classmates. Instead, we are mourning her death and we are trying as a community to understand this senseless loss of life.”

The prom, which was to be held in a banquet hall in neighboring Stratford, has been postponed.

Many girls still picked up their corsages at florists — and took them to the school gym to add to a makeshift memorial.

Other students painted a boulder near the school in purple, writing “Maren” in big white letters above her birth date.

Butts and other students attended a beachfront vigil near Long Island Sound for Sanchez in their prom attire.

Butts wore his black tuxedo and carried his girlfriend’s teal gown over his arm. Classmates then posed for a group photo as they pointed to Sanchez’s gown.

The students also released 20 purple balloons in the sky to remember their classmate.

When asked to say some words that reminded them of Sanchez, the students yelled out, “best friend,” “great smile,” and “wanted to make the world a better place.”

“She would have made a beautiful prom queen,” one pal tweeted.

And friends said she would be named queen once their prom was rescheduled.

https://twitter.com/MarcusSolis7/status/459718379499450368/