THE LAST TIME that the Agriculture Department issued regulations on the school lunch program, they were not exactly a smashing success. The department was clobbered (fairly) for reducing school lunch portions and (not so fairly) for trying to increase menu variety--you may remember the famous Ketchup and Tofu Affair. Now, the department has approached the subject again. The new regulations avoid the tricky territory of what gets served and focus instead on what gets thrown away.

"Plate waste"--as it is known in the trade--has long been an area of policy concern. In 1975, Congress went so far as to authorize a study to examine, among other things, the "cause of plate waste in the school lunch program." The cause of school lunch waste being obvious to anyone who has ever consumed one--it is known in scientific circles as the Yecch Factor--the department never conducted the study. Instead, several years later, it is moving directly toward a remedy.

The new rules would extend to elementary school children the right to refuse food that they do not intend to consume. This libertarian approach--it is known as the "offer versus serve" method--has been in effect in high schools for some time. Parents, students and schools seem to like it. Under the new rule, schools would still be required to serve a balanced lunch, and children would not be allowed to refuse more than one or two items. Students would not be charged less if they refused items.

The plan will not, of course, please everyone. Those who adhere to the "you'll-sit-there-until-you- eat-it" school of child rearing will note that, given the choice of avoiding the sight of spinach altogether, a child may well end up eating less of the stuff than if he had to stare at it on his plate throughout a meal. Others of less authoritarian persuasion may entertain dark thoughts about the uses to which the spurned servings will be put in subsequent school offerings.

Still, it seems to us that at some point you have to trust school food service administrators to have the best interests of their clientele at heart. When the Spam croquettes keep piling up in the serving bins, surely they'll get the message. Cutting down on plate waste won't solve the financial difficulties of the school lunch program caused by last summer's budget cuts--by latest count almost 3 1/2 million fewer children now receive school lunches than did a year ago. But it might mean that more program dollars end up buying nutrition for children rather than filler for garbage cans.