I believe it went back to the war where children were short of calcium in their bones due to rationing.
Not having a heart, Maggie got rid of it as soon as she became secretary of state for education.
We used to get these really cute little 1/3 pint glass bottles of milk and a short break to drink them.
It was common practise all over Europe. Even in the GDR.
The Stasi never took milk away from children but Thatcher, Thatcher the Milk Snatcher did.