I was fortunate (or unfortunate?) to get about 1000 hours of instruction given time at a Part 91 FBO which had a large rental fleet and also provided Part 91 ("non 141" in other words) instruction.
At this place, the clients were not "future airline pilots" but your typical dentist, local business owner, etc crowd.
I wore shorts in the summer with a polo shirt, shoes were of the "timberland" style variety.
In the winter (which were mild) I wore jeans and a light jacket or a sweatshirt.
Not that it had anything to do with me personally, or my dress, but we had more students at this FBO than we knew what to do with. FBO manager said "whatever yall are doing, keep doing it"
As such, the CFI's were checking out people for rentals, doing maintenance flights for the A&P next store, we took people up to look at Christmas lights (this was announced on radio ad and we got more than expected turnout), the doctor who owned that Baron in the hangar always needed to be dropped off at that airline hub city, we did that, etc.
Fond memories
Polo shirt and shorts