Even though Lisa Hark spends her days teaching medical students about nutrition, she still faces the same dilemma as the rest of us when making lunches for her kids:

Should she pack what is good for them or what she knows they'll eat? The answer, for the mother of two, is both.

Hark, director of the Nutrition Education and Prevention Program at the University of Pennsylvania School of Medicine, takes a reality-based approach to the school lunch bag.

She makes a good old-fashioned PB&J, because, in truth, a peanut butter and jelly sandwich is a pretty darn good lunch - if you make it on whole-grain bread with good peanut butter and all-fruit, no-sugar-added preserves.

You've got fiber, B vitamins and magnesium in the bread; mono- and polyunsaturated fat, Vitamin E, niacin, folate, zinc and magnesium in the peanut butter; and pure fruit in the preserves.

But don't even think about using Marshmallow Fluff. Hark scoffs: "It's pure sugar!"

Hark, who hosted the first season of Honey We're Killing the Kids!, a TV makeover show on TLC addressing the issue of child obesity, tries to find ways to increase fiber and reduce fats in foods her kids like.

Yet once every two or three months, she'll allow her third grader to take a Kraft Lunchable to school ("I prefer not to. I think they are too processed," she said, "but he likes them"). Occasionally, she'll slip a mini chocolate candy bar in her seventh grader's lunch as a treat.

"I want them to eat healthy, but I don't want them to feel deprived," she says. If you restrict everything, children will sneak the forbidden treats. "You will find candy wrappers in the bed," Hark says.

School lunches, whether packed at home or bought in the cafeteria, have long been a subject of scrutiny. "School lunches are closely associated with child health," opined the authors of the Good Housekeeping's Book of Good Meals - back in 1927. "Uninviting paper-bag lunches, prepared without much thought for the interest of which a variety of food brings to children, should be a thing of the past."

The book recommends such lunches as "Graham crackers with peanut butter, baked cup custard, apple, milk." If you substitute "pudding cup" for "cup custard," this lunch would be familiar to today's schoolchildren.

Familiarity is, in fact, key to successful school lunches, says Ellen Klavan, a California-based psychotherapist and author of The Creative Lunch Box (Crown).

"Children are not at their most experimental at school at lunchtime," she said in a recent phone interview. "They tend to be creatures of habit, and they are juggling a lot between schoolwork and their relationships with their peers."

Thus, she continued, "they want lunch to be something familiar, something comforting." They also, she says, do not want lunch to make them stand out from the crowd, or seem "weird."

That's why the kid who loves tofu at home may not be willing to eat it at school. A lot depends on the individual school's cafeteria culture, Klavan says. In some schools, a thermos of hot-and-sour soup or a cashew-butter-and-sprout sandwich wouldn't raise an eyebrow; in others, it would be the social kiss of death.

And the rules may change suddenly in junior high or high school, when a lunch that in early years would have been considered the height of weirdness - sushi or seitan, for example - may become the height of sophistication.

The parents' job (and no one says it's easy) is "to listen to and respect what their children want while honoring their responsibility to make it healthy," Klavan says.

With "weird" food out of bounds (and the definition subject to change without notice), parents may feel they are left with limited choices. But Hark believes you can begin to tweak the familiar and gradually make it more nutritionally sound.

"If your child will only eat a baloney sandwich," she says, "evaluate the sandwich." She enumerates the criteria: Are you slathering it with mayo? Are you using five slices of baloney or two or three? Is it low-fat bologna? You could be adding needless calories and fat. Are you putting the baloney on white bread? Consider whole-grain alternatives.

"I don't think you can just change their lunch all at once and expect them to eat it," she says. "Just focus on a few small, meaningful changes." If your child does - as she has seen some do while on school field trips with her children - drink a 24-ounce soda at lunch, try reducing that to 12 ounces, and then try lower-sugar substitutes. ("There are way too many kids, especially older kids, drinking sodas. We really need to help parents stop buying it," Hark says.)