But there are concerns, as the reopening comes amid a new spike in COVID-19 cases across New York City and an alarming number of positive tests among teachers.
The 300,000 kids returning to the classroom this week are in K-5 and K-8 schools, though not all students showed up at the same time as blended learning gets underway.
RELATED: Stay informed with ABC7's NYC COVID-19 positivity rate tracker
The children will split their time between in-person and remote learning, while about 48% of students will continue with all-remote learning as their parents decided not to send them back to the classroom.
For those who returned, classes were conducted in shifts so everyone can maintain social distancing guildelines.
And this all comes with controversy, though, as infection rates are climbing in some New York City communities.
The daily positive rate exceeded 3%, and schools will close if the seven-day rolling average surpasses that same threshold.
Additionally, officials say 111 teachers and staff, and 10 students, have tested positive since September 8.
The city is considering lockdowns in specific Brooklyn neighborhoods, including Borough Park, Midwood and Bensonhurst.
In addition to this concern amid students' return, the city is racing to hire thousands of teachers to adequately cover in-person and remote learning.
United Federation of Teachers President Michael Mulgrew said he was "cautiously optimistic" in the morning, and by the evening, he said the first day went "largely without incident."
"Today marked an important turning point: the reopening of hundreds of elementary schools for our city's children," he said in a statement. "We achieved this -- the first big city system to do so -- thanks in large part to the incredible efforts of tens of thousands of teachers and other UFT members who have worked relentlessly to serve their students while seeing that the children, their families and their colleagues remain safe. Today's reopening was largely without incident, and in cases where there were problems -- for example, six schools did not have a nurse -- we were able to resolve them."
He also announced a hotline teachers can call to report any issues they face, and promised "immediate action."
Meantime on Sunday, the principals' union issued a vote of no confidence in Mayor Bill de Blasio and Schools Chancellor Richard Carranza.
The union is asking the state to run New York City schools - a request which the governor has denied.
MORE: Gov. Cuomo responds to principals call for state takeover of New York City schools
"It's a big moment for the city," de Blasio said. "As many as half a million kids could be in school in the course of this week...We're here to serve kids, and I don't know a single educator who believes we can serve kids well with all remote alone, and certainly not kids with a lot of challenges and disadvantages."
On Staten Island, City Councilman Joseph Borelli says he intends to sue the school system, seeking an injunction to end remote learning and bring children back into the classroom.
He and his supporters are holding a rally Thursday, when have middle and high school students are set to return to the classroom.
Even as the majority of students can return to in person learning this week, only 52 percent are opting to do so.
ALSO READ: NYC Back to School: UFT says New York City needs 6-7K more teachers due to remote learning demands
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