Unfortunately education isn't 'a one size fits all' scenario.
The outcry from parents whose children are not in top sets, and therefore less likely to even apply to grammars, would be loud.
Schools are far too busy getting ALL their children up to the standards set down by the government. (What they do, or don't, once they get them their, is a whole other debate.)
It would be impractical to put resources into such a low return venture; as it would only benefit a few. And the thought of adding extra work, to our already highly over worked teachers, would probably send them running for the trees!
But I'm sure your (and my) school won't hesitate to mention the rather prestigious fact in their prospectus, that '9 of their children went on to the local grammar last year'!