Spending time observing a student can be challenging on many levels. In being in the room, you as an observer change the feel of the room. The behavior of the teacher and students changes.
Teachers become more aware of themselves and their teaching. They may even change some of their typical mannerisms because they fear being judged for a certain behavior. I remember something I had one professor say about her time being observed. She found it unnerving, especially when it was her superior. One cannot help be self-conscience when you know someone is watching you.
As for the students, you inadvertently become the focus, even if you try to “blend in with the walls.” I have noticed this most acutely as a student teacher. I come into the class to observe the teacher and the students turned to me, asking questions and curious as to why I am there in their room. I was glad I had come in early so that this question time did not infringe on the teacher’s instructional time. Another aspect that changes is that the students’ behavior may get worse or inexplicably improve. If they get worse, it may be because the students are vying for your attention. On the other hand, students may want to be on their best behavior, and thus not act out as they would usually.
All of this needs to be taken into account when making classroom observations for Anecdotal Observation (AO) documentation.