As for fruit, the children may also bring it from home. Sweet spreads or chocolates are normally not allowed in the matpakke, the food should be healthy and not dangerous for the teeth. In high-school, too, pupils bring their matpakke. In most schools there are no cantinas. In the professional life, you would still see the matpakke being brought along in the cafeterias.
Few people have warm meals at lunch, most have their homemade matpakke.
Dinner might be this early because it is usual to have an evening meal , and sometimes also coffee and cakes in the afternoon. Along with it may come cakes or sweets. Kaffe can be anytime in the late afternoon and early evening. It's very common to invite people over for coffee - more common than dinner, in my opinion. There isn't necessarily kaffe everyday, but often during the weekend when people have time for visiting.