...these are after-school study sessions. It's in the title of the manga. It's also explicitly stated on the last page.
Why are there just a bunch of students out in the hallway? Why is the teacher still in the "laboratory"? What is the teacher doing there? Chilling? It's fine if that's the answer.
Let me be clear that this is ultimately a minor issue. The other students could just be from an after-school club (doesn't look like it because of the amount of students and how one has a backpack, but it's possible). The teacher could just be...grading papers or something.
But the first chapters were all about how they wouldn't get caught studying like this after school, so it still seems strange.