Basic of basic = The teacher's position that can control the student's mind. (1 Approaching the speaking student.)

poko pocketさんによるイラストACからのイラスト

Sometimes, you can go nearby the student who is speaking.
You are saying "Hey Taro, you wrote a very nice opinion didn't you? Just stand up and read it aloud", while walking toward the student.
During the students read it, you are standing next to him/her nodding positively. At that time you should make eye contact with the whole class. When the student finishes reading, you come back to the front again saying like "OK everybody, Let's get writing something you think about his/her opinion".
That's it. Just doing like this will make the students focus on the speakers.
You don't have to say to the students "Listen carefully!" etc.