What an absurd question. Most of the prompt was about students, so I'll tell you how much time it costs: about three seconds for the students to see where on the boot up screen the web browser is. Then they click on the web browser, and that's it. From there it's all the same and they just go to google classroom or whatever. I speak empirically. I taught at a school that went from Windows to Chromebooks, and the students barely even noticed the transition.
Long time Slashdot reader theodp is remembering his own childhood where maybe people had to actively learn to use DOS, and then the Mac GUI was kind of a strange transition, and etc. Times have changed, grandpa!