indeed no guarantees for GS from ANY school.
If you think about it most kids are either:
-going to get in easily to GS,
-haven't got a chance or
...somewhere in the middle
Probably tutoring and primary school affect the quite large last group.
However I think you have to look at why you consider a primary school GOOD or THE BEST in an area.. presumably this is from SATS as there are few hard measures to find. How does a school get good SATS and a good reputation,
.... well it is probably in a fairly affluent area so high achieving parents who may well have passed on their higher than average intelligence genes to their offspring send the kids to the school and get good results and hence the school gets a good reputation and people look at it and want to move into the area to send the children there but it is hard to get to live there are because the houses (in estate agent speak) are "in a sought after area, with excellent schools" so the only people who can move in are affluent and successful and their kids do well so the school continues to do well...... phew it goes on.