The groupings shall be determined on the following basis:
Group A: school population of secondary age over 100,000
Group B: school population of secondary age between 60,000 and 100,000
Group C: school population of secondary age below 60,000.
A review of these figures shall take place every three years. General Committee may decide the grouping of those Counties which fall within 1,000 of these limits and which would be subject to a change of group.