These situations happen to us all at some point during our careers. Try to adjust your lesson according to the circumstances. You may decide to substitute the activity with another that you know your students can do successfully. If a student acts out, do not hesitate to implement your school's discipline procedures. Then, during your post-observation conference, explain the professional reasons for the changes you made.
Good teaching includes adaptability to a variety of circumstances.
In most school districts, the observation is followed up by a conference between teacher and administrator, usually within the next several days or weeks. This is your chance to debrief, jot down your own observations of how the lesson went so you can discuss these with your administrator. During the conference, be sure to listen and pick up on suggestions for improving your instruction. If you disagree with something, talk about it calmly. Explain pertinent background information.