At the Super Save in Helena, Mont., last week, Peggy Hunter found California oranges for 69 cents a pound. That's not surprising. We're accustomed to a sophisticated highway system linking orange groves to checkout counters. The better the highways, the lower the price.
Suppose, though, that whenever she bought an orange, Miss Hunter had to pay each state over whose highways it traveled a hefty ''user fee.'' Citrus fruit in Helena would be as rare as it was in Charles Dickens's London. In fact, however, our highways are funded substantially by state and federal taxes. Miss Hunter gets the orange. The rest of us help offset the price.
Unfair? Not at all. If my federal tax dollars lower the cost of her oranges, her tax dollars help me afford Wisconsin cheese in Boston. And if it doesn't quite balance - if far-flung Helena needs highways more than does cosmopolitan Boston - that's fair, too. We are one nation. The price of that oneness sometimes includes surrendering a strict ''let the user pay'' philosophy for a more important ''common good.''
In the realm of commerce, this sort of oranges-to-Helena argument is widely accepted. It is also accepted in matters of national defense. Helena may be a fairly low-profile military target, but Montanans still pay the same rate for the nation's military as the rest of the nation. We're all in it together: The defense of one requires the defense of all.
Why, then, has this argument been so roundly disregarded in the equally important realm of education? Look, for example, at the issue of tuition tax credits. In a press conference last month, President Reagan again called for tax deductions for low- and middle-income parents whose children are in private schools - on the grounds that they are ''also paying their full share of taxes to support public schools.''
Now, the usual argument against his position concerns the separation of church and state. It's a valid concern: Of the 4.96 million students in the nation's private schools (fall 1980 figures), 4.16 million were in parochial or religious schools. But the President's ''full share of taxes'' notion raises a different danger. The implication is that these parents will unfairly be made to pay double - paying ''user fees,'' in effect, for schools they don't use.
But what does it mean to use the public schools? As America grows older, with smaller percentages of schoolchildren among increasing numbers of senior citizens, it's a question worth asking - especially given the pattern of tough school-budget fights in local communities around the country. Nobody - not seniors, not singles, not empty-nesters - wants to pay for something from which they don't benefit.
So how do such people benefit?
The answer lies not in evening classes for adults or weekend carnivals for the community: Those are largely issues of using school buildings. Nor does it lie simply with the fact that the neighbors of schoolchildren can find responsible baby sitters or lawn trimmers. It lies, instead, in the intangibles that arise so strongly from a thriving public school - a sense of community, a respect for authority among the young, an enlightened democracy interested in its future, even a lower incidence of vandalism and other crimes - that one can hardly say that those without public-school children are not ''using'' the public schools. In fact, the nation as a whole benefits. Every time a public school produces an Olympic athlete or a top-ranked scholar, all of us reap the rewards. That's also fair: Since 9.3 percent of the nation's total public-school funding comes from the federal government (1980-81 figures), all of us have helped to pay for those rewards.
And if that logic sounds too ethereal, picture the reverse - a nation whose public schools really were funded only by users. As private schooling fueled by new tax incentives drew off the more affluent (and often better prepared) students, so the amount of tax money available to the public school would dwindle. Public education, once a melting pot, would become a cream separator: the students most needing education stuck in an underfunded center; the rest spinning rapidly away to the faster-moving edges. One has only to look at America's struggling inner-city schools and their more comfortable suburban counterparts to see the difficulties of such a two-tier system.
The danger of a tuition tax-credit plan, then - aside from very real church-state concerns - is that it would increase, not diminish, the pressure toward two tiers. At bottom, the problem lies in allowing something as simplistic as the concept of a ''user fee'' to govern something as universal as public education.
We've outgrown that notion in commerce: Peggy Hunter can afford oranges in Helena. We've outgrown it in defense: She pays her full share for national security. It's time we outgrew it in education. It's part of the price of having one nation.