Class size is an education term. There are two general uses of it.
In American high schools and higher, it is generally used to talk about the amount of students in a grade. Many larger high schools have class sizes around 400 students. Most American colleges and universities have class sizes between 800 and 10000 students.
In elementary schools and in all schools in England, class size refers to the amount of pupils in a single class. Recently, schools have tried to make efforts to keep class sizes down under 25 students.