A few questions might be:
Has the school been deducting tax in the background? Is his monthly salary a gross or net number? This could actually make the situation worse if the his salary is net as it could put his income from 15% to the 35% bracket.
Maybe his wife should get her own tax card? Her income is in the zone of 50 million, in cash mostly, meaning she isn't obliged apparently, but if his wife's income is added to his income it could put him into the 35% bracket.
Is tax actually payable monthly as some information suggests, or is it yearly in March?
If Indonesia collects taxes on global income, and getting out of the tax system is quite difficult, would that mean that if he is Canadian, and goes to teach in Japan in the future, he would have to pay 35% of his earnings in Japan to the Indonesian government? That sounds odd.
Does the school have an accountant who is doing the taxes of the teachers who do have a tax card number and is the accountant willing to do his?
What then is an estimate of the taxes that will be due at year end?
Does anyone have insight on the answer to these questions and about what might be the right questions to ask to the school's director at the meeting next week?