A selective State Boarding School is a bit of niche, currently there are two 'selective' Co Ed State Boarding Schools to choose from Cranbrook and Ripon Grammar. Sir Roger Manwood's closed their boarding house post Covid as did the wrongly titled Skegness Grammar, whose academic standards must be among the lowest 'Selective Schools' with only 66% @Gcse grade 5 English/Maths.
Hence, the average academic attainment of pupils in State Boarding Schools deeming themselves Selective is likely to be lower. I have 40 years of experience of Kent Grammar Schools though myself, sister children and cousins etc In what must be quite rare there have been 15 passes and 0 failures.
Therefore, I can comment on all the Grammar Schools in West Kent, and Canterbury with some knowledge. MY first daughter is currently in her third year at York University she went to Weald which during her years 12 and 13 had become a bit of a Shit Show with a joke of a Head and SLT making the Covid situation even worse.
I found it very sad as an ex Weald Girl myself which prided itself in offering a great education. This historically educating girls who were a grade or two below the more academic Tonbridge Grammar. DS is year 12 at Judd having doing 4 A levels attained 9 Grade 9's and 1 Grade 8 . I think on balance he personally did not want to change school though I pushed initially Ripon Grammar two years ago to both Son and Daughter no 2 . Neither child wanted to move from Kent to North Yorkshire. Daughter 2 moved from Tonbridge Grammar to Cranbrook, she would have moved in year 12 because she wants to do A Levels rather than than the International Baccalaureate . So that would have left Weald ! Judd if bright enough or Tunbridge Girls as choices for Sixth Form entry.
Regarding academic quality, well it is baffling how some boys passed the 11+ at Tunbridge Boys. I think the actual range of pupils academic ability there is far below the top 25% !