It was supposed to be one of the most important weeks of Jennifer Mejia's young life: the high school prom on Wednesday and graduation Thursday.

On Saturday night, the excited 18-year-old slipped into her blue prom gown and modeled it for her family. "She was a young girl full of life," said her father, Rene Mejia, 43, of Medford.

But Sunday, Jennifer Mejia's family said, she was one of four victims shot and killed in an apparent robbery attempt at Haven Drugs, where she worked as a pharmacy assistant just a five-minute drive from her home.

Mejia, the daughter of Salvadoran immigrants, was remembered Sunday as a hard-working Bellport High School student who got good grades and wanted to be a physician. She worked at the drugstore, said her father, because it helped her learn about medicine.

The news stunned neighbors, friends and school officials, who said the blow was all the more staggering for its timing -- just before graduation and on Father's Day. The teen's friends were waiting outside the pharmacy Sunday night to place flowers and candles in a makeshift memorial.

"Today must have been the worst Father's Day her father could have had," said classmate Kimberly Jimenez, 18. "She's going to be really missed."

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Jimenez said Mejia was not scheduled to work Sunday but was covering someone else's shift. As she rushed to work at 10 a.m., Jennifer Mejia wished her father a quick "Happy Father's Day" and said she'd be back to celebrate, Rene Mejia said.

About 20 minutes later, Mejia received a phone call from someone saying there was trouble at the pharmacy. He said he called Jennifer but there was no answer. He turned to his wife, Antonia, and said: "We better go. You never know."

Rene Mejia spoke to a police officer standing at the yellow crime scene tape. The officer left for 10 minutes, then returned and took him inside the drugstore. Mejia said he identified his daughter as one of the dead.

Her father said Jennifer was the second-youngest of four children, "a wonderful daughter" -- respectful, a practicing Catholic, a peacemaker in family disputes. She planned to go to Suffolk Community College in the fall and hoped to eventually study medicine, he said.

He said his other daughter, Leslie, 17, also worked at the pharmacy. He was thankful he did not lose two daughters Sunday. "You have to remember the good things," Rene Mejia told reporters outside his home.

Jennifer Mejia was born in the United States but her father and mother emigrated from the Santa Ana region of El Salvador more than two decades ago. Rene Mejia runs a construction business and was proud of his daughter's success and work ethic.

"She's always working and doing her schoolwork," said neighbor Damress Diaz.

South Country Central School District officials said counselors would be available for Mejia's classmates Monday.

"Jennifer was an exceptional young woman, the kind of child and student that any parent would be proud of, hardworking, intelligent and well-liked by her peers," said district Superintendent Joseph L. Cipp Jr.

Mejia's classmates organized a candlelight vigil Sunday night on the beach at the Bellport Village Marina, bringing more than 50 people together on short notice with Facebook and mass text messages.

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Teenagers hugged, cried and lit candles in the sand. "It's going to be harder walking at graduation when she is not walking with us," said Jimenez.

Katherine Rivera, 20, of Bellport, a former classmate, said Mejia "had something in all of us that brought us here together. She was kind of like the mom of everybody, always trying to make everybody happy . . . She was a good girl."

Mourners streamed in and out of the Mejia home. "She was an angel of God," said Fidencio Ascencio, 54, a family friend and neighbor in Santa Ana, El Salvador. "She didn't have to die this way."

With Mikala Jamison and Patrick Whittle