Mayor Eric Adams insisted Friday that the return to classrooms this week was “extremely, extremely smooth” — despite at least one Big Apple school being forced to turn kids away as they grappled with the influx of migrant students.

The first day of school kicked off Thursday, with 21,000 migrants inundating already jam-packed classrooms in the nation’s largest school district.

The chaos saw a Queens high school quickly hit capacity Thursday morning, sending some students to a facility across the street.

But Adams, who recently warned that the spiraling crisis would “destroy” the city, said “it went well.”

“Well, it went well, experienced well,” Hizzoner told Geoff Bennett on PBS’s “Newshour.”

“It’s always exciting, the beginning of the school year, watching the optimism, some crying faces leaving mommy for the first time. But it was extremely, extremely smooth transition ending summer,” he continued.

However, it didn’t go smoothly for Newcomers High School in Long Island City, where a line to get in snaked around the block before it became overwhelmed, pushing students at Gotham — a different school inside the building — to a facility across the street. 

“It’s a capacity issue,” one teacher griped told The Post. “They should have worked this out two days ago!”

George Kara Lekas, 14, who was supposed to start at Gotham on Thursday, said he now wants to transfer into the Academy of American Studies, “like as soon as I can!” 

Ahead of the chaotic return to schools, the Department of Education handed out just a brief memo to educators — telling them to figure it out themselves.

The 2-page missive, entitled “Guidance on Project Open Arms,” told teachers they are on their own when it comes to procuring supplies for migrants and says it’s their responsibility to find teachers who can communicate with the students, most of whom speak no English.

Still, despite insisting the start of the new school year went off without a hitch, Adams repeated his most ominous comments yet on the multi-billion-dollar crisis crippling New York City — warning that it would “destroy” the Big Apple.

“If we don’t get this right, it is going to destroy this city, and it is going to harm them. I cannot sugarcoat this to the New York City public. They need to know what we are facing right now,” he told PBS.

Hizzoner again blamed the Biden administration and reiterated calls for the White House to step up and take responsibility.

“What we believe the federal government could do is just simply one have a real decompression strategy all over the entire country so cities are not absorbing this issue,” he said.

“And particularly in those areas when you’re having these major concerns, there should be a state of emergency that the federal government is calling to make sure that we are getting the support to the cities right away.

Adams has estimated that the price tag for caring for the asylum seekers in the Big Apple could reach $12 billion by 2025. The city currently has nearly 60,000 migrants in its care.

“I say this over and over again,” he told PBS. “No city should be carrying the weight of a national problem.