The start of school will be a surprise test for New York City’s teachers and principals — as the DOE has handed out only a brief memo to educators instructing them on how to deal with the influx of migrants they will face when classes begin Thursday, the Post has learned.

The nation’s largest school district gave Big Apple educators just a 415-words letter to guide them as they welcome some 21,000 migrant kids, which is 2,500 more than the number officials gave last week, the Department of Education said Wednesday.

The 2-page missive, entitled “Guidance on Project Open Arms,” tells 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.

It doesn’t even tell local school leaders how many migrants will show up on day one.

“Principals have been preparing their schools for weeks and months and to get something two days in advance seems delinquent on the part of the DOE,” said Craig Slutzkin, a DOE parent who sits on the Community Education Council for District 2, which has saw a large influx of migrant students last year.

As Gov. Kathy Hochul warned Wednesday that migrants don’t just speak Spanish, but a multitude of other languages, the letter offers no help in bridging the language gap. It says only that principals “identify a staff member at your school who can communicate.”

Individual schools were given just ten days to identify how many English-language learners they have, or twice that amount of time if the students had special needs, according to the letter.

It also advises school staff that the students may not have school supplies, but offers no concrete way to procure them.

It instead asks teachers to consider using allocated “funds to create care closets or care packages to provide school supplies, clothing/shoes, and hygiene products/toiletries.”

The missive suggests that the DOE can’t give schools any idea of how many migrant students any one school will be taking in, because it does not track immigration status and warns that administrators “should not turn away any students.”

And if there is still any trouble, the DOE offers only red-tape, telling schools with problems to “Please complete the Central Project Open Arms Team-Support Request form.”

Staten Island Assemblyman Sam Pirozzolo said he wasn’t surprised the DOE seems to have little guidance for schools.

“How could the DOE be prepared? We have people flying in from the continent of Africa. How can we be prepared for the influx of languages?” the Republican told The Post.

“The migrant influx appears to have become an unmitigated disaster. My prediction is the DOE will be scrambling to catch up and kids won’t be learning.”

Queens Councilwoman Vickie Paladino, whose district includes PS 31, an elementary school in Bayside the is expecting to host many migrant students, said she is expecting trouble.

“The first day of school is usually chaotic. But now with the migrant crisis it’s going to be bizzaro,” said the Queens Republican.

Paladino said principals have complained to local pols that enrollment fluctuates because the migrants are a transient population who come and go. 

“A student can be in a school for a week and then go elsewhere,” Paladino said.

The guidance came a day before Chancellor David Banks said 2,500 new students who live in temporary housing had enrolled in city public schools since July.

About 19,000 young shelter residents total are enrolled in city schools, and the overwhelming majority of them are asylum seekers who had already attended school last year, officials said.

Before DOE workers did an enrollment push at city shelters this week, education honchos said there would only be about 500 migrant students added to the student roster citywide.

The title of the letter, Project Open Arms, is the name of a partnership between the DOE and city social service agencies that has allocated money to shore up classroom aid for the new student migrant population.

The city had hired 188 teachers licensed to teach English as a new language, plus another 175 new teachers who are bilingual over the last year, Education Chancellor David Banks said Wednesday.

That meant some 3,400 English as a New Language licensed teachers on hand, as well as more than 1,700 teachers that spoke both Spanish and English throughout the five boroughs, officials said.

“This year, we will continue to welcome our students with open arms,” said Schools Chancellor David Banks, who pointed out that the school system has shed an estimated 120,000 students over the last five years.

“We’re excited to fill these empty seats with new students and to bring new voices into our school communities.”

At PS 145 on Manhattan’s Upper West Side Wednesday afternoon, a teacher’s assistant who will be teaching kindergarten this year said she was optimistic the department had enough resources for the new students.

“Kids of asylum seekers are being supported,” said Riya W., explaining that the bilingual program has a lot of Spanish-speaking teachers instructing this year. 

“I’m excited for the school year, to see new kids and also see the old ones,” said the instructor, who was decorating her classroom and declined to give her full last name.

Fabricio R. and his son, an incoming kindergartener, were outside the school drawing with chalk out with other kids as part of a school tradition.

“We’re just excited to meet the teacher, you know, because he’s got a new one, and meet his classmates. Yeah, yeah, he’s actually going into the bilingual program this year in Spanish. And English, but they also have English and Russian as well,” the proud pop said. 

All city students, including migrants, were required to receive the DTaP vaccine, which protects against diphtheria, tetanus and pertussis, as well as being inoculated against measles, mumps, rubella, Varicella, Hepatitis B.

They are also required to be vaccinated against poliovirus, which the city’s Health Commissioner Ashwin Vasan recently warned that about half the almost 60,000 migrants in the city’s care had not been.

Students are not required to be vaccinated against COVID-19.

Additional reporting by Craig McCarthy