This is an excellent question. I hope someone has an answer.
If you made your own granola bars, and throw in some raisins and dark chocolate chunks, would they eat that? How about blue corn chips, or flavored organic corn chips? Those things aren't exactly the healthiest but they are not as bad as the regular versions.
Maybe you could make some cookies like oatmeal raisin with molasses, lots of butter, eggs, and pack them each one big or two medium cookies, once a week. You could make a full batch and freeze the dough in portions so you could just cook what you needed the day before.
Of course, depends on what the kids like and what they are willing to be seen with. I frankly would enjoy gourmet food everyday but I know most kids would hate that. Last year my brother was a junior in high school, he would not eat the school lunches but took all kinds of interesting things. He would take things like taco salad (with lots of homemade taco meat), spaghetti, caesar salad, hummus with bell pepper wedges and blue corn chips. His friends would always be interested to see what he would bring, but he (like me) does not like sandwiches or school food. I guess he is doing the same thing this year.
I have been playing the piano for the local high school chorus occasionally and have realized that many of the kids do not eat anything the school serves, and they also do not bring a lunch. They just eat candy bars and chips and soft drinks out of the long row of vending machines! My cousin goes to that school, she says that their food is gross, BUT they do have a pretty good salad bar so she eats that every day.
I myself struggle with what to take as a pack-lunch (and I'm an adult). Frankly, I don't like many sandwiches, only hot sandwiches. Luncheon meat and peanut butter and jelly have always been disgusting to me.
Of course, at most places you can't heat your lunch, or perhaps the place will only have a microwave which I would not use. So it needs to be something that can keep well enough for those hours... and I don't really like salad so I wouldn't want to eat that everyday. Very perplexing, indeed. So right now, I eat out at a restaurant, drive home to eat, or bring the occasional thing like I made an organic cheese and portabella mushroom "burger" which was good but too much trouble, methinks.