The acting head's permanent charge also looks on rather shaky ground come the next Ofsted - added value is almost as bad, which seems to be the trigger here.
Is it fully selective? If so it should be achieving 98-99-100% (i.e. 100% less the occasional individual lost cause), 92% is about what you'd expect for a partially selective school (it's probably beyond even the best comp though, unless in a truly exceptional year).
It is fully selective but not as selective as some of the other grammar schools in the borough (Trafford being wholly grammar/secondary). It's also not a cosy middle-class enclave kind of grammar school, either. I don't mean that to sound negative - just pointing out that it's maybe not your typical grammar school.