Advocates say free school lunch for all students fits perfectly with Mayor Bill de Blasio's agenda of battling inequality, and they are angry he won't pay for it. A new report examines just how much it would cost. NY1's Lindsey Christ filed the following report.

Nearly 80 percent of city schoolchildren come from families living below federal poverty guidelines.

Records show at least 200,000 of them are not eating school lunch.

Experts say it's because many kids would rather go hungry than admit they qualify for the free meals. 

"Along the lines of the mayor's theme, there is a tale of two cities that plays out in New York City public school lunch rooms. And that's where bullying and teasing, and the anticipation of it, prevents children who are hungry from eating," said Liz Accles of Community Food Advocates.

For years, a coalition of educators, unions and hunger advocates have urged the city to give all students free lunch. 

Bill de Blasio agreed when he ran for mayor four years ago. 

"I think we're missing an opportunity to use available federal funding to make school lunches free, and I think we have to end any stigma and universalize it. It's crucial," he said in 2013.

In his first year as mayor, de Blasio instituted universal free lunch in middle schools, but he hasn't expanded the program.  

"It's kind of mind-boggling that a mayor where this is so fundamental to his agenda. We're talking about children eating in school and families that don't have money to pay the bills," Accles said.

But a report out Wednesday by the city's Independent Budget Office says, in effect, that there is no free lunch, that it could cost many, many times more than the $3.6 million a year estimated by advocates.

The IBO says expanding the number of lunches to 90 percent of elementary school students from 60 percent today would cost $36.5 million a year.

That's free meals for 71,000 additional kids each day. 

A spokesperson for the mayor says it's still possible he could fund an expansion.

"We are reviewing the possibility of expanding it to even more schools," said Freddi Goldstein, "and are doing our due diligence to make sure resources are used wisely."

The teachers' union, which just endorsed the mayor for re-election, said it still hopes universal school lunch will get funded, even without the mayor's support. 

"So now we look to City Council to get it done, and we believe that's the avenue that we could possibly get it done this year," said Michael Mulgrew, president of the United Federation of Teachers.

But if the IBO is right, it may be even more challenging to get funding.