The positive reward system is the only thing that seemed to work with my son. Pre-school bathrooms are scaled down to be pint-sized, plus, a teacher or assistant will go with her and help.
I don't blame her for not wanting to go in public restrooms...they are big, dirty and smelly. Maybe try the kid's wing at church, the library or some place that might be less intimidating for her at first. Then reward her for good results, and positively-motivate any oopsies and make them teaching moments.
Maybe make a fun/funny bathroom song, go to the library and read the "my first potty" books again. Keep it light and fun. 4951