Our son started walking home at 5th grade when he entered middle school. It's only .6 miles in a residential neighborhood so it's an easy walk. In elementary school, he took the bus, but bus service isn't even available for middle school and high school in our neighborhood because it is so close. In nice weather, I bike to school with him and then he has an enjoyable and quick ride home. Sometimes, for a treat, I walk to meet him and we walk home together. It just helps change up the routine on occasion.
At just-turned-13, even though he walks home every day, we tend to drive him there--more because of a sleep and time issue than anything. The sun is just waking up itself when we head to school each morning. (The sun wasn't even beyond the horizon of the hills this morning when we left!)
In addition, there are no kids his age on our street and it kind of freaks him out to walk in the morning alone. You'd think that wasn't an issue at his age, but mix tiredness, no one to walk with, and the cold into it and it's a hard sell. In good weather, we walk or ride bikes together to school to provide an incentive. Sometimes, even in the winter I can talk him into walking if there is enough time and I go with him.
My preference would be for him to walk both directions all the time, but "letting" him walk does not necessarily translate into "getting" him to walk! For the first two years of middle school, my husband offered to drive our son to school when he was leaving for work. Now that his schedule has changed, I'm the lucky one. I haven't gotten our son trained yet to walk on a regular basis in the morning. Call me a wimp for not insisting, but I'm working on it. As he gets older, I'm expecting him to take more initiative...to walk or ride. We'll see how that goes.
P.S. I walked to school every day of my life--rain, sleet, or snow--except for one year when we were bussed to another school while ours was being expanded. Otherwise, in grade school, I walked roughly 1/2 mile with friends. In high school, we walked 1 mile with a group of 2-4 kids.
In general, I think walking or riding bikes is the best solution. And the buddy system is preferred, if other kids are available.