Because the classroom is the school split by subject, or by a lesson, but the actual work is included in many subject lessons gathered together, which are even more complex.
The classes will be devoted to a separate topic or lesson. But the real work is to gather together many lessons, which are more complex. And some operations may be different from what I have learned.
Because the classroom is a study in separate topics or according to lessons. But the real work is to collect several lessons together, which have become more complicated.