The custom was well established before my era 1943-1949 but practised in a different manner from the 1960s.
On the last Sunday of each term, as we skipped from the church pews into the aisle, each girl in turn would give the wooden threshold to the pew a good backward kick with the heel of one shoe.
There was therefore a dull continuous clatteration until all had left the church. Of course those of us sitting near to members of staff could only touch the pew silently as the practice was frowned upon.
The later method of everyone kicking at once must have been introduced to prevent the long drawn out kicking of earlier years.