One of my drawing instructors insisted we take our 18x24" newsprint pads sketching in public, so that we were absolutely forced to endure the attention, the distractions, comments , whatever, so we could develop strategies for dealing with the public and also develop the discipline to maintain focus on drawing in less than ideal circumstances.
I've never had a negative reaction from anybody who noticed me sketching them, in one instance at least quite the contrary. I was in one of the university libraries on the second floor sketching people at tables on the first floor for perspective/foreshortening practice. A young woman I'd been drawing as she bent over a book at a table suddenly sat up straight and stretched her arms way over her head, really arched her back, noticed me above, smiled, and, um, arched her back a little bit more.
"Three's so little room for error."--Elwell