I recently spent a year as a full-time language student in a college (in China, so more like US high school in many ways) after teaching English for 10 yrs, and

I recently spent a year as a full-time language student in a college (in China, so more like US high school in many ways) after teaching English for 10 yrs, and also unexpectedly discovered point no. 1, i.e. how tiring it is to sit down all day, and almost never did so as a teacher. Even after 1 year I was still often yawning after 2 or 3 hrs of near continuous sitting, and always got up to walk around outside for 5 or 15 minutes during breaks. Most other students were much younger, usually 20-ish, and often stayed sitting in class during breaks. I don't know how they do it, but maybe it's just their age, or maybe they haven't done a job where they're on their feet all day for 10 yrs.