Principal scales are quite wide, in terms of pay they stretch through the equivalent range of MX8/9/10 in the civil service depending on the complexity and type of school they are heading. While a normal non-scholar will definitely need to be an identified high performer to reach principal, it is by no means reserved only for scholars.

In fact most scholars will not even go through a stint as a school principal as they advance to senior levels within various ministries including MOE. For e.g. the 2 top civil servant PS in MOE now were not even teaching before and were simply rotating within many portfolios in multiple ministries and SAF.

Overseas scholars like SAFOS or those in AO scheme or equivalent have entirely different career tracks, cannot use basic "farmer" sort of linear concept like Teacher -> SH/LH -> HOD -> VP -> Principal to evaluate.