School lunch programs vary by school, district, and often influences of parents. The National School Lunch Program (NSLP), established in the late 1940's, has been important in helping provide nutrition to our kids. Guidelines for the foods provided by this program, through the USDA, have been updated several times. Recently, the NSLP has taken steps prohibiting serving foods of minimal nutritional value (including fruit drinks and sodas) in the food service area during meal periods (see link below).
Meals provided by the NSLP are overall balanced, but one problem is that the children eating the food may leave portions, sometimes for example the fruit (healthier portions) and only eat the other foods. This is very difficult to control. Certainly parents can encourage their children to eat breakfast (some schools now provide breakfast as well), and often send healthy lunch options from home.
Parent involvement can be very effective. Simply regulating extra food (e.g., what is sold at 'bake sales', or substituting fruits for pizza/ice cream parties or rewards) can be effective. Parents (e.g., in San Francisco) have also helped advocate for healthier foods in the schools, resulting in changes in the types and ways foods are provided to the children.
I would contact your school for information as to where you can look up the nutritional content of school lunches. It's hard to give a blanket "yes" or "no" answer. Some days the lunch might be more unhealthy (pepperoni pizza) compared to other days (chicken salad), but it also depends on your definition of healthy. There are lots of things to look at such as fat, calories, sodium, processed ingredients, etc.
I'd figure out what criteria makes food "healthy" in your eyes and compare what you would pack at home for your child versus an average score of what the school has available. You can then make an informed decision as to whether it's worthwhile for you to pack a healthier lunch for your child or not.
To have the good handle on what's going into your kid's belly during school hours, your best bet is to pack his or her lunch, fill it with fresh, wholesome, and nutritious ingredients -- and avoid processed foods and junk foods. For smart school lunch ideas, have a look at Education.com's article on Healthy School Lunches:
You have already got good answers here, I provided another link to education.com, where you can just scroll through a list of past articles on the subject, all are interesting.
I've also included another link that you may want to read over, it has the actual recipes and preparation facts for the food, this will give an idea of "what" they are serving and "how" it should be prepared. If you see a recipe you like to try at home, you can log in and scale the servings to fit your family and it will generate the new recipe.
As for my school, I don't know that it's a matter of the lunch not being healthy as it is the menu itself. If they are serving something that your child will not eat, then yes, it was not a healthy lunch. So, I mostly pack lunch, at least they will eat and it's actually more economical to pack the lunch. I provided a link to Stayfit containers, these are the containers that I use. You place them in your freezer until ready to use. Place your sandwich, salad, etc. in the container and it will keep it fresh without having to pack a bulky ice pack in the lunch box. Then I can place a juice box in with the sandwich container and it also keeps the drink cold. These containers are wonderful. They also have one that you just put water in a microwave for a few minutes, pour the water out and add soup, pasta,etc. to send a warm lunch and they come with a spoon right in the lid, they really work, and I am able to provide a warm lunch on cold days,
I know you've seen the "lunchables" kids love so much. Use one of these containers and make your own to save more money, I use cookie cutters to cut the shapes from deli ham, cheese, etc and send crackers.
If you are interested in the containers, shop around for the best price and I have seen them at Wal-mart. Hope this was helpful!
As long as you pack healthy foods your lunch from home will be more appetizing then the school lunch. Schools must meet certain guidelines for meeting the four food groups but that doesn't mean your child will choose the best. Many times schools will serve macaroni and potatoes on the same day. Hamburgers can be soy burgers.