I cant see how the minor change would affect people too much. You are obviously willing to take all the kids to school with you. I think 30 days notice is more than enough, if you give too much notice they will just forget. i would do the 30 days and then a remind the week before and suggest that they start the new schedule that week just to get into the new routine. Just be sure to make sure that parents fully understand that if they dont arrive by a certain time that you will be leaving for school. When i did school runs parents used to arrive 2 minutes after my "set" time to leave and would wonder why I wasnt there LOL.