If the city and its teachers union do not agree soon on new teacher evaluations, class sizes will likely rise, teacher training suffer, after-school activities be eliminated, and guidance counselors cut, Chancellor Dennis Walcott predicted this morning.

Walcott spelled out the doomsday scenario during a brief talk about teacher evaluations at the Manhattan Institute this morning. He said he had called UFT President Michael Mulgrew — at 7:50 a.m. today — to say he wanted to conclude negotiations by Dec. 21, or two weeks from Friday and the last regular workday before Christmas.

Reaching an agreement by Dec. 21 would give state education officials, who have expressed increasing anxiety about the city’s timeline, nearly a month to review the plan and request any necessary adjustments before a deadline that Gov. Andrew Cuomo set last January.

State education law requires that districts adopt new evaluation systems when they next negotiate contracts with their teachers unions. But Cuomo vowed to withhold increases in state school aid from districts that do not have evaluation systems in place by Jan. 17, 2013.

In a statement, UFT President Michael Mulgrew said there was no need to commit to a Dec. 21 agreement and said politics were again impeding the union’s good-faith effort to negotiate new evaluations.

“Rather than establishing bogus deadlines and threatening parents with the loss of teachers and services, they should be focusing on reaching an agreement that will actually help make the schools better,” Mulgrew said about city officials.

The $250 million that Walcott said is on the line represents 4 percent of the nearly $8 billion that the city’s schools receive from the state each year. It represents a far smaller share — about 1 percent — of the city Department of Education’s total operating budget, which is about $23 billion annually.

Still, the amount is larger than in most rounds of budget cuts that the Department of Education has experienced in recent years. Those budget cuts led to a reduction in the city’s teaching force, larger classes in all grades, and cuts to extracurricular programming, according to a union survey conducted last year.

Exactly when and how deep the state’s cuts would come is not totally clear, but city Department of Education officials said the funds are already included in this year’s school budgets and would be cut midyear without a deal. Midyear budget cuts are doubly disruptive to schools because most expenses are fixed for the whole year, meaning that only certain costs, such as after-school programs or tutoring, can go on the chopping block right away.

Lawyers familiar with school funding are looking into questions about whether Cuomo can legally withhold the funds, according to David Sciarra, a lawyer who is representing city parents and advocates in a renewed push to secure funds for high-needs districts that the courts have said the state must provide.

But even if the threat turns out to be legal, it is unsavory, Sciarra said.

“At the same time that the state has walked away from its obligation to fairly fund the poorest schools in our state, including the schools here, to then turn around and use the threat of withholding funding for those very same programs that kids need — frankly it’s unconscionable,” he said.

Sciarra noted that the federal government is allowed to withhold Title I funding from states that do not comply with its mandates. But, he said, “they never do that, because they know when they cut that funding, the kids and schools that would be hurt the most are the most at-risk.”

Here’s what Walcott said in his Manhattan Institute speech about the possible consequences to city schools of losing out on the increased school aid:

While we will look for savings centrally, as we always do, we know we will not be able to absorb the entire $250 million.

If we can’t reach an agreement with the UFT, we will be forced to pass some cuts on to schools.

At this point, any cut to our schools is too much, especially when you consider the structure of our schools’ budgets.

Schools spend over 95% on personnel costs and the majority of the remainder on direct student services.

Any cuts will undermine exactly what we are trying to achieve – providing meaningful support and development opportunities to our educators and rigorous instructional programming to our students.

While principals would make final decisions about how to absorb budget reductions, we would expect that cuts would lead to fewer teachers being hired, which will probably lead to larger class sizes.

We would expect the elimination of professional development opportunities for staff and cuts to cherished after school activities such as music, art, and sports for students.

We would expect substantial reductions in guidance counselors, social workers, and other support staff who play a key role in our children’s social and emotional development.

We would expect schools to stop purchasing instructional materials such a library books and educational software.

This is an unfortunate reality and these cuts would be painful.