Below might be of varying interest to you depending Provision for different MFLs, music, art, drama computer science at GCSE and A level. 3 or 2 year GCSEs (i.e. selecting options in year 8 or 9) and if the former how do they extend the curriculum. Any house system with activities and competitions. Sports. Support for students falling behind (results may be more about support by parents and tutoring than teaching)
Are the kids pushed too much? What is the staff turnover? (Not specific to grammars that one, but can be great indicator) How do the GCSE options work? Some schools effectively only offer one 'free' choice. I would be cheeky and ask "would you send your child here?".