Since there won't be any seat belts on the bus, I'd agree to leave the car setas at home.
But, please reconsider your 6 year old not using a booster seat. He/she may not be big enough for just a seat belt yet. Many parents allow their kids to stop using a booster too early because the kids "don't want to sit in it".
From the American Academy of Pediatrics web site:
"Remember, seat belts are made for adults. If the seat belt does not fit your child correctly, he should stay in a booster seat until the adult seat belts fit him correctly.
This is usually when the child reaches about 4' 9" in height and is between 8 and 12 years of age. "
Link:
http://www.aap.org/family/carseatguide.htm
If the shoulder belt is hitting the child's neck or throat or the lap belt is across their stomach they are too small to ride without a booster. My child is 4' 4" and I just double checked the fit with and without a booster. The booster is definitey safer because it keeps the lap belt lower to the hip area instead of across the stomach.