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: