How routinely did Albus Dumbledore wander the halls, the classrooms, the common rooms of Hogwarts--invisibly? Could Mrs. Norris see him? How is that different from the way we supervise students in school with cameras, data-gathering instructional apps, even just watching them in the classroom? For better or for worse--students are watched and supervised and even surveilled all the time they're in school, maybe not by anyone invisible--but we all accept that as normal.