Did she really say it was the most exhausting or is that how interpreted it from "only teachers would understand"? While it's massive generalisation from her part, half the people i know think teaching is lots of fun and ohhh the holidays and ohhh working school hours only.
However the job is physically,mentally and emotionally draining. I know teachers that arrive at 7 am and leave at 6pm, and that's without clubs,meetings,seminars for parents, parent teachers meeting etc. Then there's the planning,sorting out or going to buy resources,marking,making displays,sorting out the xmas play(this term at least) and other events. And that's without the actual teaching,challenging behaviour,kids crying,kids refusing to do work,the kids that you spent a whole term teaching and can't even tell instantly that there are 5 fingers on one hand. If in ks1 there's also reading,spellings,handwriting that are separate but need to be fit in during the school hours,along with the curriculum. And john lost his hat,anna isn't getting money for the non uniform day and she's sad, jamie is hungry and his tummy hurts and can't focus because there was no breakfast,catherine is feverish and poorly but mum can't come pick her up,the kid that is behind emotionally and socially is winding up the kid with anger issues,the mum of a kid is aggressive at pick up and right in the TA's face and you have to calm her down, ofsted are coming and everyone is on your case about how everything looks and not how things are,you ask for help and get snapped at and to deal with it and so on and so on. And that's just in one day.
And no i'm not a teacher,tbh i could never be one.