For scores of children and parents, being assigned to Sandra Santos-Vizcaino’s third-grade classroom at Public School 9 in Brooklyn felt like winning the lottery.

Though Matteo Flores was only in the first grade this year, he was already hoping she would be his teacher in two years. Then he heard the news. “I guess she won’t get to be my teacher anymore,” he told his mother.

Ms. Santos-Vizcaino died on March 31 of complications of the coronavirus, her son, Victor-Luis, said. She was 54 and is believed to be the first public-school teacher in New York City to die of the virus, according to the Department of Education.

Ms. Santos-Vizcaino spent four years as one of the most beloved instructors at P.S. 9, in the Prospect Heights neighborhood, part of her 25-year teaching career in New York City, which is the country’s largest public school district.