If it's not acceptable to show children up when they struggle with maths, it's equally unacceptable to show them up for not being able to run fast.
OMG People actually believe that!
it's the complete opposite, children need to know where they are, they need to learn that being at the top requires work, if not from others, but from their part.
The children are not even ranked between themselves, the marks are only based on an average. Your child is not 5th, or 17th or 28th in the class, which would be fine, he's just into one of 3 categories.
I am amazed that Grammar Schools are still legal with that kind of attitude. Kids need to know that they need to work to be first, they won't be the first all the time, and if they are really crap at something they should work harder and be proud of the area where they are better.
The standards are not so high in Primary (and Secondary frankly) that all kids are not able to succeed. Some might need help, tuition, extra work at home, but there's nothing they can't do. Parents are doing them a huge disservice by pretending it's not true.