Well I guess the norm is that after the school year ends, children get a long summer break and resume a new year after the break instead of returning for the remaining year after a long summer break.
I didn't get how bullying is related to when you start the school year.
I'm taking a stab at it, but if I'm not mistaken, in the west (look at their seasons), back in the olden days, kids were needed to help out on the family farms. As a result they were unable to attend school during the summer months which is why the school was not in session during the summers.
I don't know why the school year begins in May in Pakistan.