Please can someone tell me what makes the real difference between underperforming schools and great schools.

There may be several reasons why a school does or doesn't perform to expectations. Perhaps the parents are not involved in their children's education as shrryslone suggests. Maybe the learning environment is too disruptive to allow kids to learn effectively. Maybe there are some teachers that aren't good teachers. Perhaps there are a high number of students who are new English-learners, who may not score as high on tests as do the native-speaking kids. There could be a dozen reasons.

I suggest you meet with the school's principal and get his/her explanation for low test scores. Hopefully you'll get a reasonable explanation, but more importantly, the recognition that something has to change. If the administration knows there's a problem, but doesn't make changing it a priority -- THEN you know you have a problem at that school. If, however, you have administration who recognizes the issues and has an active plan for improvement, you're in a much better situation.

Finally, armed with the information about the principal's explanation and plan, check with some other parents -- ideally ones who have been there a while, -- and see what opinions you get from them. That may help reinforce the message and impression you get from your principal, or, at least, will likely provide you with more information you can use to make a sound decision for your child.