Why are Belgian taxpayers funding schools on a different continent in the first place?
Because children in undeveloped countries have a lower chance of becoming terrorists or economic migrants if they can get a half-decent education. Foreign aid is not a hippy-dippy guilt trip, it's an international stability booster, and very cost-effective compared to military intervention.
The Belgian government has no business extorting its citizens' income to finance the lives of people who live in a different country. The Belgian people will never get any benefits for their tax dollars pouring into a foreign country. Their tax money should be used to fund their own children's education.