It is advisable to first enter into a discussion with the teacher or managing board of the school. If you want to file a formal complaint you can do this via the school board.
In Flanders, if you have any questions regarding this you can also contact the CLB: http://www.onderwijs.vlaanderen.be/centrum-voor-leerlingenbegeleiding-clb
In Wallonia you can contact the Centre PMS:
http://www.enseignement.be/index.php?page=24633
Complaints can also be sent to: Directorate-General for Compulsory Education by e-mail: plainte.dgeo@cfwb.be or by post DGEO - rue Adolphe Lavallée 1 to 1080 Brussels