Where in the United States Constitution does a give the first lady the power to dictate school lunches?
That's a good question Oliviaforever. And the answer is that it doesn't.
In fact, the Supreme Court has officially clarified that the Constitution's silence about any issue, school lunches for intrastate schools in this example, means that the federal government is prohibited from interefering with such things.
”From the accepted doctrine that the United States is a government of delegated powers, it follows that those not expressly granted, or reasonably to be implied from such as are conferred, are reserved to the states, or to the people. To forestall any suggestion to the contrary, the Tenth Amendment was adopted. The same proposition, otherwise stated, is that powers not granted are prohibited [emphasis added].” —United States v. Butler, 1936.