They've started to show up on city sidewalks: kids wearing backpacks and school uniforms early in the morning and late in the afternoon.
The first day of classes for the city's traditional public schools is September 5, but most of the 236 charter schools across the five boroughs open in August.
In Harlem, Monday was the first day of classes at the Neighborhood Charter School.
The 8,000 students who attend the Achievement First network of 23 charter schools in Brooklyn have been back since August 15.
A record 120,000 students will attend charter school students in the city this year, an increase of more than 6,000 over last year. And the number of charters, 13 more than last year, is also a record.
Charter schools are publicly funded but privately managed. Many of them believe in extending the school year, and the school day.
"We truly function under the principle that the most precious resource that we have is time. And we believe deeply in maximizing that, both by potentially having a slightly extended school year but also by an extended day," said Caroline Roth, principal at Achievement First Endeavor Middle School.
The School Day at all of the Achievement First middle schools begins at 7:15 a.m. and ends at 4:40 p.m. The charter network says students who attend Achievement First schools from kindergarten through 12th grade attend the equivalent of an additional school year thanks to the longer day and year.
"The earlier we start, the better," Roth said.
Students say the long intense days, and homework at night, can be trying, especially when their friends in traditional public schools are still in vacation mode.
"We got to be here learning while they still playing basketball, football, dancing. It's tough," said Makhi Crosland, a student at Achievement First Endeavor Middle School.
Most of the charters spend a lot of time telling students why it's important to have an extended school year, an explanation the students repeat, although some sound a bit skeptical in mid-August.
"It's going to pay off because you get more education and you can learn more stuff in the school year than at most other schools," Crosland said.