During the 50s, 60,s and 70's admission to the school was dependent on success in an examination generally called the 11-plus exam, or sometimes the scholarship exam. This examination consisted of written papers in Arithmetic, General Composition, Essay, and a test in Mental Arithmetic. We were schooled hard in our final years at Primary School to face these exams. The exam was held in February and the results were published in the Aberdare Leader at the end of May. 

1952 11-plus pass list
Including all schools in the Cynon Valley.
From Drew Haman.
You can view the 11-plus exam papers taken by Drew at his web site here.

1952 letter from the Local Education Authority
From Drew Haman

1957 11-plus pass list
Including all the schools in the Cynon Valley.
From Colin Rees.

Mental arithmetic
Some sample mental arithmetic papers set at primary school to prepare us for the exam.
From Colin Rees