*Surely driving instructors have access to a car, and tend to live local to their students or if not work from a company base.
Therefore there is absolutely no reason why any driving instructor wouldn't have a poo either at home or the office.*
I used to be a driving instructor. Unlike many others who work in small to medium towns, or rurally, I operated near the centre of a very large city, one which was not built for cars, and this was a real problem. Loads of pupils living in the city centre would want lessons, because most instructors wouldn't venture there at all. There would be vast areas completely without a public toilet quickly accessible from the road, and I mean completely. Most petrol stations closed theirs because of drug users; pubs or even McDonalds would be difficult because of parking (and in a large city, parking attendants would pounce quickly); supermarkets with car parks were rare; and slow city traffic meant any driver could only cover short distances, so no time to nip home or to the office (many driving schools don't even have an office); and as every self-employed person knows, the more people you see in one day, the more money you make: no time for detouring via home for a pee break. I marked the very few public toilets feasible for road use on my map.
So, I frequently used to ask pupils if I could use their toilet before or after a lesson, for weeing and pooing, and I make no apology for it. I tried not to do so with the same pupils too often. Only one person ever refused; she said the house wasn't tidy enough.