Chances are good that your two children will have minimal interaction within the confines of their school as they are several grades apart. The bus ride to and from school, however, is a different story. I simply would let them both know that you have instructed the bus driver to relay any misbehavior among the two directly to you. There will then be consequences at home for misbehavior because the bus driver needs to pay attention to the road for safety and not break up any sibling arguments.
I also would reward good behavior with extra privileges for acting well in public. Most importantly, I would let them know that socially appropriate behavior is expected whether you are with siblings, friends or strangers. Acting impolite, rude and mean is not an acceptable behavior. If you also set up these parameters at home, then this should decrease negative behaviors, too.
Children also learn by example. If you tend to argue with the children or others in the home then they will learn that this is an ok way to interact with others. Therefore, reasonable modes of communication should be employed, whenever possible.
Here are some resources I recommend: There is an excellent book that I would like to suggest, "Siblings Without Rivalry" by Adele Faber and Elaine Mazlish. This book clearly explains the most common reasons why older children become jealous and act out when a sibling comes along. Also, strategies for coping and changing behaviors are spelled out in this book. These authors also have a companion book titled, "How to Talk So Kids Will Listen & Listen So Kids Will Talk".
Get Out of My Life, but First Could You Drive Me & Cheryl to the Mall: A Parent's Guide to the New Teenager, Revised and Updated by Anthony E. Wolf Ph.D. and Anthony E. Wolf also is a good resource.