PoA_PRC8/_module/nss/mageonly.nss

void main()
{
object oDoor = OBJECT_SELF;

object oPC = GetNearestCreature(CREATURE_TYPE_PLAYER_CHAR, PLAYER_CHAR_IS_PC);
int class1 = GetLevelByClass(CLASS_TYPE_WIZARD, oPC);
int class2 = GetLevelByClass(CLASS_TYPE_SORCERER, oPC);
int pass = class1 + class2;
if (pass >=20){

    AssignCommand(oDoor, ActionOpenDoor(oDoor));
    SendMessageToPC(oPC, "You may pass.");
}
else{
SendMessageToPC(oPC, "You will not pass.");
}
}