Saadia, if the school has 3 classes (@90 children) in each group each child is 'worth' just over 1%. In a school where there is 1 class (@30 children) each child is 'worth' 3.5%.
So, the results in smaller schools can be more skewed because if 1 child is ill, away or has SEN needs, this will affect the results more- I actually think smaller schools have a harder time because of this.
As a teacher I don't believe it is right to focus on a few statistics which are based on specific circumstances, e.g. mainly on cohort. I prefer to consider pupils and their results as individuals- did the results show a true percetion of their ability?
Have a look at the Value Added score (VA) which should be next to it, this shows how much the school has moved them on. This still needs to be considered carefully however as children move schools so don't necessarily have the same teaching experience, or have other circumstances which affect their results.
Any school would be very aware of a change in results...believe me they would most likely have known about it before the SATs tests and they would be looking at reasons to why it happened, the LEA and Ofsted ask these questions too. But if you are concerned I would speak to your teacher/head.