The guidelines applies to secondary school education; primary schools may follow at a later stage. However, it will be up to each individual school to decide whether they adopt the system or not.

French, English or German: these languages are only spoken during the specific language courses in secondary school. But this will change as from the next school year. "Our education system should prepare children for the future. And this future is multilingual", the Flemish Education Minister Pascal Smet explains. Still, pupils will always have a way out if they wish. If a school decides to have the geography classes taught in French, then this course should always be available in Dutch as well.
The "Content and Language Integrated Learning" project is actually similar to the so-called "language immersion schools" in Wallonia, but there up to 75 percent of the different courses are not taught in French. Another difference is that the Walloon system applies as from primary school. Barbara De Groot of the Brussels university VUB told VRT radio that "the Walloon system has a very positive impact on pupils’ language skills, also on the mother tongue. It’s a false assumption that the mother tongue would be affected as a result."
Flemish schools will be able to make their own choice whether or not they adopt the Flemish guideline.
[Flandersnews.be / Expatica]