SCHOOLCHILDREN should eat lunch every day -- there is no doubt about that. Nor is there doubt that many spoil their appetites by stuffing on candy and other junk food before lunch. But is that sufficient reason for the federal government to instruct the principals of 90,000 schools, and the school boards that supervise them, to lock up the vending machines every morning?

The Department of Agriculture thinks it is. It proposed a regulation on Thursday barring schools that accept federal school lunch money from permitting the scale of candy, chewing gum and other foods of "minimum nutritional value" until after lunch. Assistant Secretary Carol Tucker Foreman explained that the federal taxpayers spend about $3 billion a year on those lunches and "gave a right to expect them to work well and to contribute to their children's well-being."

The Department of Agriculture tried this same kind of thing last year. Its proposal then was to bar "junk food," but that failed because there was no general agreement on what "junk food" actually is. By changing the wording to "minimum nutritional valve," the department thinks it has solved the problem and is on its way to improving the health by changing the living styles of thousands of American students. It also thinks -- and it may be right about this -- that what it is doing Congress told it to do in 1977 when the legislators gave the department power to regulate the sales of "competitive foods" near school lunchrooms.

The real question, however, has nothing to do with "competitive" or "junk" or "minimum nutritional value" foods. It is whether the federal government has any business telling local school authorities what they can sell in their schools and when they can sell it. Those are questions local school boards are competent to decide without any help at all from Washington or from Uncle Sugar, as the old rescal used to be known.