While this would indeed distort the player’s rankings, I think the overall system would suffer more if unranked was set as a default setting, resulting in many more unranked games and a less accurate ranking system - simply due to inertia.
We would probably also get a number of newcomers to OGS who are wondering why they are still ? ranked after playing 10 games and not noticing the checkbox.
I think the actual solution to your issue is to introduce a special “teaching game” option which is unranked and specific to this type of game, including more tools available to the teacher to guide exactly how the teaching game progresses. This feature has been proposed and discussed here in the past.
https://forums.online-go.com/t/feature-request-teaching-mode/34832