i don't know what the rules for the public schools are in the state we live in now, but the public schools in our former state would require medical documentation and some kind of plan put in place to admit a child who was not entirely potty trained. the issue had to do with health and safety laws-as a teacher i was strictly forbidden by law to assist a child with anything dealing with bathroom issues so unless a plan was in place to deal with accidents and proper storage of soiled clothes (or disposal of pads and such), as well as clean up (if leakage occured) a kindergartener who did'nt have a documented medical issue would have been denied entry (or if we did'nt find out about it prior to admission, disenrolled).
i think the great 'out' that state had as many others do with kids that grade/general age is that kindergarten is'nt required (so the state is'nt in violation of a law if they deny enrollment) and/or the usual age for kindergarten is below the age the state is mandated to being obligated to school a child (k was'nt required in our former state/usual age for k was 5 but attendance is'nt required until age 6, in the state i'm in now k is'nt required/usual age for k is 5 BUT attendance is'nt required until age 8

(yup, and believe me there are people who don't send kids to school here until they get close to that age, that's why dd's highschool classmates look like college students).