Depending on the province though, kids might have little more than a lunchbox with bread slices with butter and ham, which doesn't seem very enjoyable to me (I'm French, with kids in Belgium).
It has to be cold and eaten with the hands, and "not too special", anything other than sliced bread with some kind of meat or spread inside is explicitly frowned upon.
All schools in West Flanders do provide lunch though, and at least some in East Flanders too (obviously we only looked up the areas that matter to us and we didn't conduct an exhaustive study though). So we just concluded that it depends on the province.
At the very least it does not depend on the school system or the region since it varies within Flanders and within the Go! system.