When I was in school, we never really made trades. Rarely anyone brought their lunch; it was all the school lunch, and you had no choice on what you got. Even if you didn't want that frozen peach cup in heavy syrup, you were still given that frozen peach cup in heavy syrup.
There were times where it was, "Are you going to eat that?" If the person wasn't going to eat it, whoever asked for it got it.
You had more freedom in what you got in middle school. Lunch at my middle school was all a la carte. They'd have frozen burritos that were heated up, steamed hamburgers with mushy buns, or fries for the most part. And sodas. Sometimes, they'd have Domino's Personal Size Pizzas. That was pretty much all you got to choose from. The Student Store had Cup of Noodles and frozen yoghurt.
At my first high school, my group of friends and all shared. Some days this group of people would buy food and we'd all eat from it. The next day, this group would buy food and we'd all eat from it. Again, the variety was mainly frozen burritos, hamburgers, and corn dogs. There was a side window that sold fruit salads and Domino's Personal Size Pizzas. Then we had a Student Store that sold chips, candy, and soda.
My second high school had meals like my elementary school. You were given certain things, no choice against it. They were also really stingy on condiments. That was the only "trading" I ever did. They gave use one packet of ketchup and one packet of mustard on Hamburger days. I don't like ketchup, so I usually gave that to someone else and everyone who didn't like mustard gave me theirs.
My last high school had a mix of a meal with certain things and no choices against it, or they had an a la carte line. I usually checked out the other line. I'd get a plain milk (not chocolate, that's gross) and a Chick-fil-A sandwich, though I didn't know it was a Chick-fil-A sandwich at the time; I just knew it was chicken.
I eventually grew tired of the school lunch and started just bringing peanut butter sandwiches, granola bars, and peanut butter crackers with me to school. I always ate the sandwich at lunch time. The granola bar and crackers I just ate whenever during the day.
I guess we were boring.