Yuck- kids can be incredibly mean.
I would definitely make an appointment with the principal- please don't just "stop in". Make sure that the secretary has you booked on the calendar and confirm it with an email or a hand-delivered note. In your "confirmation note" mention what you would like to discuss.
Truthfully, the school social worker did what she could b/c she doesn't really have the authority (in most schools) to apply a consequence. Also, don't assume that the principal is aware of what has gone on. You say "they are telling me"... in reality it sounds like the social worker is telling you so remember not to generarlize your statements.
Your son should not be intimidated on the bus- the girl is bullying him. Ask the principal about the district's anti-bullying policy and don't leave the meeting until there is a plan in place to make sure that your son is not physically or verbally harassed on the bus. It is not uncommon to have "assigned seats" on a bus to avoid these situations.
After your meeting with the principal, follow up with a short "thank you for taking the time to meet with me..." note that also outlines the "plan" and your assurance that you will keep her in the loop should something happen.
Keep copies of your communication in a folder at home just in case! Schools can't solve all the problems of the world, but we do have an obligation to create a safe learning environment for our kids!