Parents ask to know more about schools. So why are they forbidden to observe the classroom?

He was intrigued by the contrast between what he had heard about certain teachers and what he saw in class. “In some cases, in my opinion, teachers with extremely positive representation are barely qualified, albeit entertaining, and teachers with dull or slight negative, in my view, did an excellent job,” he said.
https://www.washingtonpost.com/local/education/school-parent-classroom-observations-visits/2021/11/19/b40fb28e-47b6-11ec-b05d-3cb9d96eb495_story.html Parents ask to know more about schools. So why are they forbidden to observe the classroom?