Re: School lunches
You husband should have been the first one to advice against keeping the food warm for that long. (He is an engineer and interested in science too) Warm provides right environment for bacteria multiplication. Keeping it warm for a half max one hour is fine but for many hour is an invitation for disaster.
Bacteria, or other germs, need time, food and moisture (or wetness) to grow; but they won’t grow when the temperature of the food is colder than 41º F or hotter than 140º F. The temperatures in between 41º and 140º are in the “Danger Zone.” Keep potentially hazardous foods out of the “Danger Zone!” For example, when food is left in the “Danger Zone”, bacteria can grow fast, and make poisons that can make your customers and family very sick.
Read more here.
There is a reason we keep food items in freezers and refrigerators.
When you warm up the food it is best to heat it up to the temperature it was cooked at.
So cold sandwiches are your best bet.
For elementary school kids they do not let the kids use microwave on their own and teachers do not have the time to do it for them.