I imagine in Yr 7 they will just be end of year tests set by the school themselves and they will be aimed at a child of grammar school ability. I don't think it would be reasonable to compare them to something like SATs where you might expect a child of this ability to achieve 80%+.
My son, who is Yr 9, took SATs recently and is now sitting end of year exams set by the school. They still have to do maths, english and science as the school feel that the level of the SATs is not high enough to really test their knowledge.
So to answer your question, these tests are not really going to tell you how well the school is performing. The results achieved will depend entirely on the difficulty of the exam, but if my child was achieving 80% against an average of 70% I would be quite happy.