Make sure that everyone who participates lives relatively near each other. Including one parent who lives out of the way can cause resentment.
Assign one parent to act as group leader - the person who will handle scheduling as well as alternative plans to account for sick drivers or sick kids.
Decide whether everyone will drive equally or whether some will make up for not driving by pitching in for gas and maintenance.
Determine each child's drop-off and pick-up sites and times.
Draw up a car pool schedule. You can have one parent take the kids to school and another pick them up, or have a rotating schedule in which each parent has car pool duty for a week or a month.
Check that each driver has appropriate insurance coverage.
Make and distribute a list with all parents' names and phone numbers.