RE Educating GS kids and SM kids in the same building ...
Going by the description of my neighbours who send their kids there, the HT at the local non selective is very anti competition. Prize Giving Day was watered down because he didn't think that it was right that the same high achievers kept getting up to accept the awards. As for sports, the school football team kids are swapped in and out of the A, B and C teams, regardless of ability. Why? So no child feels a failure for always being in the C team.
Contrast that our selective. Prize Giving Day is one long boring protracted affair what with the award for x the Netball Team captain, award for Y for her contribution to the school music scene and the Mrs Huffington-Smythe award for an outstanding Year 11 essay on xyz. Then there is the focus on getting as many students as possible into Oxbridge or failing that, into a RG uni.
If you merge the two schools together the two ethos (I've no idea what the plural of ethos is)will be in conflict. If your child is 'average' then do you want to hear your HT banging on about Oxbridge, child x who won the local Maths Olympiad etc? Conversely, If your child is exceptionally bright, do you really want to listen to the HT go on about how there is more to life than Oxbridge and a highly paid City job and how we shouldn't single out the winner for attention blah blah blah.
No one suggests merging Oxford U with Oxford Poly (or whatever its called). Yet merging GMs and SMs is popular with some people. Strange.