It is difficult to have students that are exactly at the same level in a class. I've found it impossible so far,  because I've got few courses, it's not a big school with several different groups, therefore I try to group the people coming to my classroom in the best way I can.
Moreover, even if they have the same score in the same entry test, they might then be different in class.
This difference is due to personality, previous experiences and different personal skills. There is a person with a very good memory, another one who can make sentences in English without wondering whether they are right or wrong, and succeed in communicating, there's another who can write perfectly but cannot say a word unless it's written down on paper. There's someone who can understand when I speak, no matter how fast I'm speaking,  and someone who understand songs better than any speech.
I think having students with different abilities in class is not a disavantage, on the other way round. It enriches the class, as far as students are able to gain from listening to others, to recognize their own ability and are not frustrated by their own limits; it should be the goal of each student, not trying to be better than the other student, but trying  to overcome their own limits.
And I see this as a rule in life, not just in class. :-D