I think that riding the bus is fine for K. I would definitely do it if the other choice is driving.
But walking to school is a really lovely way to spend a little time with your kid, and start the day with fresh air and sunshine. I really loved walking my kid to school. So, if walking him those 20 minutes is an option, even just some days, I'd vote for that.
Luckily, with public school buses you can just make the decision day by day. Raining? On the bus you go! Early work meeting? Climb on!
Coming home, it's better to be consistent so they don't confused and do the wrong thing.