Education leaders Tuesday swiftly condemned President Trump’s decision to wind down a program that offered protections for undocumented immigrants who arrived in the U.S. as young children, warning the move will disrupt the lives of hundreds of thousands of students.

The Trump administration announced it would end the Deferred Action for Childhood Arrivals program in six months, giving Congress a chance to address the issue. The program, introduced by President Barack Obama in 2012, offered work permits and a reprieve from deportation to 800,000 undocumented immigrants who arrived in the United States as children. Many DACA recipients are high school and college students who face uncertain prospects as they head back to class.

Daniel A. Domenech, executive director of the national association of school superintendents, known as AASA, said his members supported the immigration policy and are “concerned by the uncertainty today’s announcement brings, not only to our students and their families, but also our broader schools and communities.”

Of the nearly 1.2 million eligible for DACA by 2014, the Migration Policy Institute estimated that 365,000 were in middle and high school, and 241,000 were in college. Teachers and administrators say they fear that ending DACA could disrupt the education of students, and have sought to reassure them they are safe at school.

Critics of the program said Obama overstepped his authority when he created the program and that it allows undocumented immigrants to take jobs away from U.S. citizens and legal residents.

But education leaders and groups from across the country assailed the Trump administration for ending a program that paved the way for undocumented immigrants to attend colleges and universities. Hundreds returned to school to get their high school diploma or GED because the program required it.

Some DACA recipients became teachers. Teach for America, the teacher-training corp, has 100 DACA recipients in its ranks. Denver Public Schools, where more than half of students are Latino, said the move could be devastating for its teachers who are DACA recipients.

In Denver, more than 1,100 high school students walked out of class to protest Trump’s announcement, joining a massive rally at the Auraria Campus in that city. High school principals also joined the assembly, said Denver schools spokesman Will Jones. Jones said the district began recruiting DACA recipients to teach because it wanted its classroom teachers to reflect the demographics of its students.

John B. King Jr., who served as education secretary under President Obama and is now CEO of the Education Trust, an advocacy organization, called the move “irresponsible and immoral.”

“DACA has benefited communities, schools and colleges — but most importantly, this program has helped students by giving them the chance to attain a higher education so they can build a better future for themselves, their families and the country they love,” King said.

Randi Weingarten, president of the American Federation for Teachers, said Trump is breaking his promise to treat DACA recipients — widely known as “dreamers” — “with great heart.”

“Betraying DACA dreamers is betraying the values of our diverse and welcoming nation. America will not be stronger or more secure when these young people are torn away from the country they love and call their own,” Weingarten said. “As children return to school, many carry with them constant, crippling terror and uncertainty because of their immigration status. Children should be free to learn and live without fear. Inhumane immigration policies deprive them of that freedom.”

Education leaders have warned that if Trump pulled the plug on the program, fearful students might stay home.

“A lot of these kids might start going into hiding,” said Robert Runcie, superintendent of Broward County Public Schools in Florida. “There’s going to be a lot of fear and uncertainty for young people.”

The Council of the Great City Schools, a nationwide coalition of large, urban school districts, called on Congress to extend protections for “dreamers.”

“The mission of public schools is to create opportunity — not for some children, but for all,” the coalition said in a statement. “For urban public schools, whose classrooms are filled with students from all over the world, our mission is not to reflect or perpetuate the walls that others would build.”

“In the spirit of this mission, we condemn the dissolution of the Deferred Action for Childhood Arrivals program — whether now or in six months — by the president, and the value system that led him to conclude that America could only be great again without the patriotism, ingenuity, and voices of these children.”