I know three teachers well (Chicago area; two quite senior, the other recently retired). Hours might have once diminished over time as you suggest, but their experience as told to me contradicts this trend: curricula have been changing so quickly in response to administrative decree that every few years everything gets thrown out and rebuilt. When you add to that decreasing resources for classrooms and increasing student-to-teacher ratios, it is not surprising to me that they work more hours than anybody else I know.