Teacher workload:
In high schools, it is important to consider not only the number of students per class but the nature of the class, and the subject the teacher is teaching. For example, a math teacher might have no problem teaching an advanced math class, or several math classes, with 35-40 students. But an English teacher teaching four classes of 40 students would probably not be able to give the proper attention to written assignments from that many students, and might not give as many assignments because of the large number of students.
Team teaching:
Some schools might have classes of 40 taught by a team of two teachers. The class size by itself is not necessarily an indication of the attention students are getting.
Volunteers:
Some schools effectively use upper-grade students as volunteers in the classroom. This type of instructional help may not appear in a school?s data about class size.