Would it not help perhaps by contacting a local University/College and asking them? I know some may want to shoot you through their recruitment procedures...
An idea maybe to contact High Schools and find out directly or who they report their results to...
Alternatively look for the Education Board and see if they publish that data or not... [Tried it but no luck, try using a language translator to perform the search...]
This information might get you started perhaps...
https://en.wikipedia.org/wiki/Belgium
https://en.wikipedia.org/wiki/Education_in_Belgium
https://en.wikipedia.org/wiki/Academic_grading_in_Belgium
https://en.wikipedia.org/wiki/List_of_universities_in_Belgium
https://en.wikipedia.org/wiki/Education_in_the_Flemish_Community
GL!
