* Grade level: Elementary classrooms are generally smaller than high school classrooms.
* School district: Funding and resources differ widely across districts, impacting classroom size.
* School type: Public, private, and charter schools may have different average classroom sizes.
* Subject: A science classroom might be larger than a language arts classroom to accommodate equipment and activities.
* Building design: Older buildings may have smaller classrooms than newer ones.
While there's no definitive national average, studies and reports often cite average *student-teacher ratios*, which aren't directly equivalent to classroom size but provide a related metric. These ratios frequently range from 15:1 to 25:1 or higher, depending on the factors listed above. A higher ratio generally implies a larger classroom (or more likely, more students in the same sized classroom).
To find a more specific average for a particular area or school type, you'd need to consult data from that specific school district or a relevant educational research study.