In a couple of weeks, Hall County Schools will be rolling out some green school buses amid its fleet of bright yellow vehicles.The school system has added 20 buses powered by liquefied petroleum gas, more commonly known as propane.The new buses should hit the streets by the first of May, making Hall the first system in the Southeast to use propane-fueled buses.“We’re very excited about it,” said Jill Corbin, a veteran bus driver who will operate one of the new buses. “We think they’re very nice, and they’ll be the first green buses on the road. And we’re excited about that.”The buses, school transportation officials say, have many advantages over traditional diesel-fueled buses.“You save on fuel, No.