Swapping out students with a running engine, while safe if performed correctly (and depending on aircraft type), is not really the best practice when teaching students. It needs to be emphasised to anyone new to flying that being a pilot is not just about handling the aircraft in flight but rather all aspects of safety around the aircraft when airside. That includes the safe startup and shutdown of the engine as well as the use of mags, throttle, mixture and pitch to manage the prop. By simply turning up to a running aircraft and jumping in the student is missing a lot of the learning experience. Not a good idea in my opinion.