It suggests that during the course of the previous two weeks the principal has visited each cl;ss at least once. It may mean "exactly once" rather than "at least once". It is possible that the principal has started on a round of visits intended to include each class, but has not at the time of writing completed it, but I think that would have peen phrased differently. If the round of visits had been started but not finished, I might have written "over the past two weeks I have been visiting classes" or perhaps "over the past two weeks I have been visiting classes with the intent of going to each". Or other forms of expression could have been adopted for that case.
The same Idea could have been expressed as "over the past two weeks I have visited each class" with no significant change in meaning. I would say that the difference is one of style. The form using the continuous tense suggests a process more. Perhaps in this case that the Principal's visits were part of a longer process of monitoring the condition of the school, but that is speculation.