There has been discussion in the media and among the board about the PPS rule that grades cannot be entered as less than 50%. So presumably, even if a student refuses to do an assignment at all, that student will receive a mark of 50%.
As an alternative, how about the "makeup" policy that some schools have- ie, work below a certain grade can be "made up" for a mark equal to the orginal grade and the makeup grade. For ex, a student with a 40 who got a 90 on a makeup would have a grade of 65. This way the students who were really making an effort and correcting their mistakes would be rewarded, while those who refuse to do the work at all or who simply cannot do the work would, as is appropriate, repeat the class.