It is a mandatory part of the school’s curriculum to learn Dutch in Brussels. Private schools follow other rules.
In Wallonia, it is compulsory to learn another language. The school can organise lessons in German, English, Flemish, Italian, Spanish etc. In Flanders, French is compulsory until the end of secondary school. In the German speaking Region, it is compulsory to learn French or Flemish.
Please check with the principal of your school to know what the situation is.