If you have been teaching for a number of years, it is often pretty easy to hazard a guess at the children who are candidates for grammar school. This doesn't mean that they will reach the required mark in the test, just that they would get on well in a grammar school environment.
Where we are, there are quite a few schools and each one is slightly different. A child who is suitable for one grammar school may not be suitable for the other as they focus on different things.
Your school should know what the test involves and what the grammar schools are like, in order to give the best possible advice to those parents who aren't a sure whether to put their child through or not.
A teacher won't look at a class and say who will or won't pass but they do (in our county at least) have to mark all children, before they sit the test, on how likely they are to cope in a grammar school. They will look at how they work within class, their attitude to learning and their performance. This has to be done so that if they don't get through the exam and have to go to appeal, the county can use these additional measurements.
For example, if a school is saying that a child who did scored 120 is really suited and another child who also scored 120 isn't, then the county will use the school's grading to make the decision.
It's incredibly difficult for the y6 teachers and something I know they take very seriously. It isn't a perfect system, that's why there is a 12 and 13+ but as people who spend a great deal of time, in an academic situation, they are the best people to decide.
What the school should not be doing is teaching for the test. They are administrators.