Michelle Obama said she'll fight to prevent Congress from rolling back changes to school lunch standards across the country, telling a small group of reporters that folding now is "unacceptable."

"Of course it's hard. That we expected," the first lady said of getting school cafeterias and students on board with healthier fare.

"We expected those challenges, particularly among our oldest kids who've grown up eating junk food," she added. "But what we did not expect was for the grownups to go along with it and say, 'Well, this is too hard and it costs too much money so let's stop even though we have 90 percent compliance.'"

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Obama's Healthy, Hunger-Free Kids Act drew overwhelming support when it was introduced in 2010. But a small percentage of schools said they were struggling to implement the new dietary requirements, claiming menu items rich in whole grains, fruits and vegetables were more expensive and less appetizing to students.

The House Appropriations Committee voted in May to allow schools to temporarily opt out of the school dietary requirements - a move interpreted by some as the first step toward an overhaul of the standards.

"Would you ever say, 'Let's just stop now and start over because we have 10 percent left to do?" Obama told reporters, including ABC News' chief health and medical editor Dr. Richard Besser. "That's where we are right now and that's just unacceptable."

But some critics say the first lady's stats are inaccurate.

"The government continues to perpetuate the fiction that 90 percent of school cafeterias are successfully meeting the federal nutrition standards being debated by Congress. This is unfortunate and irresponsible," Patti Montague, chief executive officer of the School Nutrition Association, said in a statement.

"Because while it's true that the vast majority of schools are in compliance with the 2012 standards, few schools are prepared to successfully meet the standards that take effect on July 1," Montague said.

"School Nutrition Association does not want to gut the current standards. Our members simply want relief from some of the onerous regulations slated to take effect this summer, which will lead to fewer students receiving healthy school meals, more food being thrown away and many school meal programs in financial straits," Montague added.

The first lady acknowledged that most schools want help implementing the new standards, not an overhaul. She also said that she thinks members of the SNA "are surprised that it has gotten to that point where their small complaints have turned into a discussion about completely overhauling standards that they had fought for in the he first place."

Obama said the standards are "critical to the ultimate success of a generation of young people.

"We just want to make sure that decisions about our kids school lunches, where taxpayers spend billions of dollars a year, that it's based on science and not on politics," she said.