By Christina Hager

BOSTON (CBS) – It was a back to school celebration with very mixed emotions in Boston. The city began the first phase of its hybrid plan Thursday with 3,500 high needs students heading into school buildings. “I’m still a little nervous, but as long as he’s doing OK, that’s all I care about,” said Anastasia Aponte, as her son walked out of the David A Ellis School in Roxbury. “We did a lot of fun things,” he reassured her.

Just one night earlier, Boston moved into the high-risk red zone on the state’s color-coded map of COVID-19 rates. “If these numbers go up, then what happens is we potentially can’t go to the next phase of school,” said Mayor Marty Walsh. “The kids that came here today are potentially at risk of not coming back.”

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A student is welcomed back to Ellis School in Roxbury (WBZ-TV)

Wednesday night, school nurses spoke at a virtual Boston School Committee meeting. “I do not believe our schools are ready to open,” said nurse Heidi Winston. Their biggest concern is ventilation. Even though school officials say they spent the summer upgrading buildings, a Boston Teachers Union Report shows problems with windows and air quality. “A partially open window in conjunction with a box-fan would actually pose risk for recirculation of a respiratory virus as rampant as COVID-19,” said school nurse Heidi Winston.

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Boston Teachers Union President Jessica Tang echoed those concerns Thursday. “As we think about the weather getting colder, we really need to think about air purifiers,” Tang said. She fought back tears as students walked into the Ellis School surrounded by applause from staff. “It is an emotional moment,” she said.

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The reopening plan for Boston Public Schools is supposed to be gradual, with several phases over two months.

Christina Hager