Hello Dear,

CBSE has not made compulsory but the school and the principal who makes it compulsory to give pre-boards and make false rule that you will not get admit cards or you will not allowed to sit in exam.  According to my opinion dont miss pre board as it helps you to get ready for the main exam. If the reason is valid then its ok but as you know pre board are very important for your practice because it gives you a real time practice.

Good Luck