Before the Second World War the grammar schools were mainlu used by the children of professional middle class - and the bright children of the working class.
As many small firms were lost during the war years, so more jobs became avavilable in larger firms. We can see that this trend has existed until today. When Woolworths closed thousands of jobs were lost. Large chains with hundreds of shops have closed down.
To get a good job in a chain or a large organisation a school leaver had to have a good education. Grammar school boys and girls were prized employees - especially in the adminstrative and management posts.
Most early grammar schools had a house system - with prefects and compulsory games.
Some things haven't changed much!