Until the age of 18 you are obliged to attend school. However, from the age of 16 (or 15 if your child has completed the first 2 years of secondary school, regardless of whether he/she has failed or passed) your child can choose part-time education in combination with working.
For more information: https://onderwijs.vlaanderen.be/nl/leerplicht-van-6-tot-18-jaar