Back in the day (when mine were just starting out) it used the be the MOE's thought processes that if a local were married to a foreigner (westerner) the odds were that it was a local girl and that the girl would follow the male spouse, therefore, needlessly taking up a valuable seat in their overcrowded schools (back then - that's also why they ran schools in two sessions). So if the 'mixed' family wanted their kid in an
International School the government reasoned why not, they will be leaving in a couple of years anyway.
That has changed somewhat and more and more westerners here are marrying local AND taking up PR and even citizenship so now it may be a little more difficult unless your child is 'British' or has 'other' lesser special needs.