不要找托辞，否则逃得了这次逃不了下次。就直接说：

Car pool means paticipating families taking turns for the pick-up／ drop-off responsibilities.  It's understandable once in a while one family can not  fulfill their duty.  But I don't feel comfortable that I am the only one driving our kids, I feel I am taken advantage of.  Unless you are willing to share the responsibility, I think it is better off you search for other options. 

ＢＥ　ＦＩＲＭ！