Go to this link: http://www.maxpreps.com/polls/football/xcellent25.htm
Under MaxPreps Freeman Rankings, click on the link for "View High School Football Rankings". On the next page, select "Texas" and also select "Division TAPPS II 6 Man". Also make sure you select the button for 6/8/9 Man. Click the button "Load Rankings". This will give you a rankings page, for which one of the columns is Strength, which I believe is strength of schedule. Not sure why some of them are negative numbers, and can't find a good explanation for how it is determined.
