student general assembly; all-student meeting
A school-wide meeting of the entire student body, often organized by the student council. Used in junior high and high school contexts.
At tomorrow's student general assembly, we will discuss the budget for the cultural festival.
生徒会 is the student council itself, while 生徒総会 is the general meeting of all students, often run by the student council.
Compound of 生徒 (student) and 総会 (general meeting).