Observations can be one of the best non-instrusive methods available. As such, you are more likely to have access to "normal," everyday behaviors that other methods interrupt. You can observe behavior while you are "under cover," i.e., participants are not aware they are being observed (i.e., you are doing your normal activities in the classroom but observing students/groups for particular behaviors or reactions) or you can let students/teachers know they are being observed and give them a general idea about your interests. There are, of course, different reasons for different approaches and different ethical considerations. The following resources help to explain when observations are appropriate and useful and what ethical issues you need to consider.
Chapter on observational research, professor of industrial design, Eindhoven University of Technology
Overview of observation process with sample guides: University of Wisconsin Extension
Chapter on observational research from Trent Focus for Research Deveopment