Many learners these days show this attitude of cutting the teacher's speech or suggesting what he should say next in his talk.
No doubt that this is from bad manners.
The Messenger of Allaah (sallaLaaahu alayhi wasallam) said:
ليس مِنّا مَن لم يُجِلَّ كَبيرَنا، ويَرحَمْ صَغيرَنا، ويَعرِفْ لعالِمِنا.
'He is not part of us, the one that does not give regards to our elderly ones, or shows mercy to our young ones, or does not give the due rights to our scholars.' (Saheeh Jaami, 5443)
Scholars who teach etiquettes of learning do say: It is not proper from the student to cut off his Shaykh in his speech until the latter finishes from the matter he is addressing.
Al-Imaam al-Bukhaaree - rahimahullaah - has a chapter in his Saheeh: Chapter of He Who Is Asked about an Aspect of Knowledge while Busy but Completes His Speech (Before Responding).
In the above report, the Messenger of Allaah (sallaLaaahu alayhi wasallam) was giving a speech when a Bedouin cut in, but the Messenger of Allaah (sallaLaaahu alayhi wasallam) continued with his speech until he finished and asked from the people: 'Where is the fellow that asked about the Hour?' Thus the Messenger of Allaah (sallaLaaahu alayhi wasallam) clearly heard him but ignored the man as a kind of lesson to him.
Similar to cutting the teacher from his speech is this common practice (sometime condoned by the teachers themselves) where the students around a teacher in a programme or a lesson will be making all sorts of comments over what the teacher is saying in a way to give more colourations to the teacher's speech. This is not proper.
May Allaah guide all of us.