what is the very difficult point while teaching in a group?

The very difficult point while teaching in a group is difference in level of understanding or ability to catch up the speed of the explanation.
It creates problem for maintaining the standard of the class.
Another difficulty is the thinking level and perception of the class varies in a group.

المستويات المختلفه للمتعلمين وعدم الفهم السريع الذى لا يتماشى والمجموعه

When I first saw "Entre les murs" / "The Class" (2008), I though it was only acting, not reality.
But then I bought the book and leart more about François Bégaudeau (see it, read it, please).
I guess there is a certain "point" (as Binod Timsina wrote it in the question) when its more like a cutting edge, a climax: you win it or you lose it (the group, I mean).
It's magic through acting.
It's making a point out of your teaching :)

1.
to motivate the students and take them on board with you.2.
to have full knowledge of the student's psychics.

The difficult but not very difficult point while teaching the group is to make consensus of different minds.
The ability of good teacher then plays a vital role to make the students understand and gather them at focus conclusion at the final statement of the teacher after discussion.

1.
To keep everyone focus from all cultures, background and many more2.
To stay confident in your teaching and keep it intresting3.
tO MAKE IT EASY UNDERSTANDABLE FOR ALL LEVELS OF PEOPLE IN THE CLASS4.
Also the most difficult thing is everyones IQ is not the same so your level of teaching have to be on a standard that will be taken in with the entire class