School lunches, oh the memories. Instant mashed potatoes served up with an ice cream scoop...watery room temperature corn niblets...rectangular pizza about the size of a pocket paperback book...and a little carton of chocolate milk.
Dinner/Lunch/Supper - the current convention where I live is to have breakfast, lunch, and dinner (and we did have lunch ladies at school when I was a kid).
For my parents' generation, it was breakfast, dinner, supper, with dinner being the big meal, served midday. My family were all laborers (farmers, mostly) and the midday meal was the big meal, and the evening meal was to just something smaller.
When work moved from the farm to the factory, the midday meal got smaller, because you had to pack it and maybe you had just 30 minutes to eat it, so the big meal shifted to the evening, and dinner replaced supper, and lunch replaced dinner.
Our eating habits through the week are a small, cold breakfast (usually cereal), a small, cold or hot lunch (like a sandwich or a slice of pizza), and a main/big meal in the evening. Weekends, it's a big, hot breakfast, a very light, cold lunch (more of a snack, just cheese and crackers), and the third meal can go either way, though it's usually something cooked.
Between-meal snacks are usually something like toast or a pastry between breakfast and lunch, a piece of fruit between lunch and supper, and a proper dessert after supper.
My parents staunchly use breakfast, dinner, and supper. If you mix those up when talking to my dad, he won't acknowledge you. I, being the bridge-builder that I am, compromise by saying breakfast, lunch, and supper.
My nieces all laugh at me for saying supper. To them, it has nothing to do with meal size, it just sounds like an old-fashioned term that Grandpa Simpson would say.