A child is very impressionable both in grade School and in middle School I think. The drama in middle School may be more visible but there is another type of drama occurring all the way from infancy that in crucial in how a child develops and behaves towards others. How a human interacts with other humans especially in the early stages of life can greatly shape the personality of a child all the way into adulthood. Attachment theory tells us that there are four main Groups of attachment that a child can fall under: secure attachment, ambivalent attachment, anxious-avoidant insecure attachment and disorganized/disoriented attachment.
Depending on the attachment early in life you'll be able to detect a pattern in the child as they grow.
So the drama you might find in kindergarten and grade school is more likely to give you pointers on the type of attachment the child has and the needs of the child. This is also the stage for a child to test boundaries. As they grow older and become more independent ( as in they can go potty alone, they can use a knife and fork, tie their shoes etc) the testing of boundaries will also increase, as well as the awareness of differences which may result in teasing of children who look, smell, behave differently from themselves.
In conclusion I'd say that middle School is where the drama might be the most visible, in terms of teasing and nasty behavior.
At the same time I'd say that the type of attachment a child has in grade School and earlier in life ( i.e. the absence or presence of good caregivers) is a crucial and sometimes quite a dramatic part of a child's life as well, because it determines so much of the person they'll develop into.