The mother of 15-year-old Nisa Mickens — one of two teenage girls slain last week in Brentwood — vowed Wednesday through tears to not let her child die in vain.

Elizabeth Alvarado said she plans to protest pervasive gang violence by sitting in front of the White House until President Barack Obama is “sick of seeing my face.”

“If you’re strong for me,” Alvarado, 48, told hundreds of mourners at her daughter’s funeral, “this is going higher than it’s supposed to go.”

Family members, friends, local elected officials, and former classmates and teachers came to pay their respects at Rose’s Funeral Home in Bay Shore. Some wept; others knelt beside Mickens’ closed coffin and prayed.

Large bouquets of flowers were placed at the front of the room, along with a balloon shaped like a basketball — Mickens’ favorite pastime.

Mickens and her best friend, 16-year-old Kayla Cuevas, were beaten to death last week, Suffolk police said. As of late Wednesday no arrests have been made.

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The girls — both high school juniors — were found dead about 24 hours apart in the same Brentwood enclave near Loretta Park Elementary School. A law enforcement source has said police are investigating a link between the killings and the MS-13 street gang, suspecting that an argument between Cuevas and gang members may have sparked the slayings.

Overcome with emotion, Alvarado was briefly escorted out of the room at one point during Wednesday’s service before returning to accept condolences.

Later, she draped herself across her daughter’s coffin and cried. “Come back home to me please, baby,” she said.

Richard Loeschner, principal of Brentwood High School’s Ross Center, told mourners that out of the school’s 4,500 students, he and Mickens greeted each other about three times a day.

“It says something about someone who distinguishes themselves like that,” Loeschner said. “I only have an opportunity to meet kids in the hallways and the cafeterias, and I know Nisa very well. Her smile lit up the room.”

The principal added: “And as you can see by the outpouring of students and families of Brentwood High School, she meant a great, great deal to us, and we will miss her dearly.”

About 120 Brentwood High students attended the funeral, each taking time to console Mickens’ parents.

Many of those who spoke at the service echoed communitywide calls to stem the violence plaguing Brentwood.

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Police officers in marked vehicles kept watch outside the building on Fifth Avenue. Members of the Guardian Angels were also on hand.

Cremation was to follow the service.

Visitation for Cuevas, held later in the afternoon at the Michael J. Grant Funeral Home in Brentwood, was just as emotional.

A group of Brentwood teachers could be seen sobbing into each other’s shoulders in the parking lot. A young girl being consoled by a friend wailed: “It’s not fair!”

Visitation was to continue Thursday from 2 to 4 p.m. and 7 to 9:30 p.m. The funeral will be held at 10:45 a.m. Friday at St. Anne’s Roman Catholic Church in Brentwood.