if the class leaves by 5:00 am and return by 10:00 pm, that means they spent a total of 17 hours both on the road and at the destiantion. If it takes 4 1/2 hours to get to Austin, then it would take another 4 1/2 hours to return home. This means they are spending 9 hours on the road both on their way to Austin and on the return journey.
Please note that 5:00 am to 5:00 pm is 12 hours, and then, from then till 10:00 pm is another 5 hours, making 17 hours in total (12 + 5)