First week will be the hardest if they are the quite shy type, My daughter didnt like being left at school and cried every time I left for about the first week but my son on the other hand couldnt get in there quick enough. it depends on how they relate to there teacher as well as it is a big step for kids that have only been with family and then are left in the hands of a bunch of stranger.
I now bribe them with a trip to 7/11 on most days and they cant get in the car quick enoungh, if they get up late and we are running late for school no toy from 7/11.
I know its probably not really good but hey it works