I have a few friends that are teachers and most of their schools wouldn't allow for this sort of thing to happen. Teachers aren't allowed to sit at their desk while a class is in session, they must be instructing or walking around (during a test, video, etc...). They get a planning period, where they might have a chance to do some grading or lesson planning, if they don't need to meet with a parent or something. This means they need to either stick around school several hours after it lets out to grade work and do lesson plans, or bring it home and work on it that night.