Attending school is free of charge. No enrolment fees can be claimed for students in nursery, primary and secondary schools in public schools. However, there are fees required for extracurricular activities, food and childcare outside of schools hours (during the morning before the school starts, lunch or after the classes have finished).
All private schools have enrolment fees. For more information: http://www.belgique-infos.be/official-school-or-free-school/?lang=en#.WQ8ZFuXyjb0