“Abbott Elementary” is a late arrival to ABC’s slide timetable — and it helps make a solid situation to stick all-around in primetime for a when. The mockumentary-fashion comedy, prepared by and starring Quinta Brunson (“A Black Woman Sketch Show”), revolves close to lifetime at an inner-metropolis elementary university in Brunson’s indigenous town of Philadelphia — notably its staff of harried instructors and its unqualified principal, Eva Coleman (Janelle James), who’s there only simply because she caught the university superintendent in a compromising place. Abbott Elementary is staying filmed for a documentary about about “underfunded, inadequately managed community universities in…