The number of S1 classes is altered between 4 and 5 on alternate school years since there would be inadequate space to accommodate 30 classes if the School accepted 5 S1 classes every year.
There is a fluctuation in the number of S1 categories from 4 to 5 every other school year, as accommodating 30 classes would not be feasible if the School were to admit 5 S1 categories annually.