The MPA rule regarding the requirement for FIS certified/stamped helmets is for both events Slalom and Giant Slalom even though FIS/USSA only requires them for GS and higher speed events (SG and DH).  FIS/USSA does not require them for Slalom, but the MPA is taking the high road on head safety and concussions.  So, for now and likely at least for this season, the rule stands, FIS certified/stamped helmet is required for both high school competition events.