For instance, if there are 100 in a class and only 20 students are eligible and all get 100%, the school will show as 100% even though 80 pupils do not have results!
Sadly no , the percentage of children is worked out using the number of children in the class. So if there are 20 children in class and only 15 sit it, even though they may all get level 4 and above the % of children level 4 and above is 75%. That's why schools ask that all children attend during SATs week.
As for choosing a primary school, is your son starting in Reception this year? If so , a lot can change in a school in 7 years. I would concentrate on finding a school where you think he will be happy. Happy children learn far better.