You can do something like this :
order=data is what you have to add.
descending makes your lowest level the last group.

See the example: You need to change the bold words.
proc sort data=YourData out=New;
      by descending YourVariable;
      run;
   proc mixed data=New   order=data;
      class YourVariable;
      model DEP= YourVariable/ solution;
      run;