I'm at school for 12 hours a day, but I am not buying outside meals every day.
I do bring sandwiches, rice/meat in containers, etc. It takes a while to prep those in the morning though, at least for slow hand me. I'm going to start bring some tamales too, because I can just keep them frozen in my backpack and microwave them whenever I eat - extremely convenient for me, because I don't have to limit myself to the fridge at school. However, tamales are not very filling and not that cheap.
What other stuff can I bring that's convenient like a tamale?