I am on Rippetoes starting strength. I'm in a weight training class at my highschool (monday, wed, fri). I have enough time on monday's and wednesday's, but not on fridays (periods are shorter on Friday's). No way to get more than 6-7 sets done on friday. What are some things that I can do to get a good workout on Friday in 25minutes? I was thinking of something with KettleBells, or maybe a tire (there's a 300 pound tire in our weightroom). What do you reccomend?